#SingleInstance Force
#NoEnv
StringCaseSense Locale

ES_NOHIDESEL := 256  ; показывает выделение текста в Edit при неактивном окне
EM_SETSEL := 0x00B1  ; установка выделения
EM_SCROLLCARET := 0xB7  ; сделать видимым выделенный текст
EM_GETSEL := 0xB0  ; извлечь позицию каретки ввода
VarSetCapacity(start, 4), VarSetCapacity(end, 4)

Loop 12
    Text .= "Как можно сделать поиск в поле Edit?`n"

Gui, -DPIScale +HWNDhGUIMain
Gui, Add, Edit, x0 y0 w500 h100 vMainEdit hwndhMainEdit %ES_NOHIDESEL%, %Text%
SendMessage, EM_SETSEL, 0, 0, , ahk_id %hMainEdit%
Gui, Show, w500 h300, Edit find

Gui, 2: +Owner1 -MaximizeBox -MinimizeBox -DPIScale +HWNDhGUIFind
Gui, 2: Add, Text, ym+5, Что:
Gui, 2: Add, Edit, x+25 yp-5 w284 vFind hwndhFind, Можно
Gui, 2: Add, Button, x+10 yp gGoSearch, Найти далее
Gui, 2: Add, Button, xp y+5 wp gCancel, Отмена
Gui, 2: Add, Checkbox, vLoop y+0 xm Checked, Зациклить поиск
Gui, 2: Add, Checkbox, vSens y+10 xp, С учетом регистра
Gui, 2: Add, GroupBox, x+10 yp-35 h55, Направление
Gui, 2: Add, Radio, xp+10 yp+25 vDirectUp, Вверх
Gui, 2: Add, Radio, x+15 Checked, Вниз 

SearchEdit: 
    Gui, 2: Show, , Найти 
    Return
    
Cancel:
    Gui, 2: Hide
    Return

GoSearch:
    GuiControlGet, DirectUp, 2:
    GuiControlGet, Sens, 2:
    GuiControlGet, Loop, 2:
    ControlGetText, MainEdit,, ahk_id %hMainEdit%
    ControlGetText, Find,, ahk_id %hFind%
    SendMessage, EM_GETSEL, &start, &end, , ahk_id %hMainEdit%
    If DirectUp
        f := (f := InStr(SubStr(MainEdit, 1, NumGet(start)), Find, Sens, 0)) ? f : (Loop ? InStr(MainEdit, Find, Sens, 0) : 0)
    Else
        f := (f := InStr(MainEdit, Find, Sens, NumGet(end)+1)) ? f : (Loop ? InStr(MainEdit, Find, Sens, 1) : 0)
    If !f
    {
        MsgBox, 8256, Edit find, Не удаётся найти "%Find%"
        Return
    }
    SendMessage, EM_SETSEL, f-1, f+StrLen(Find)-1, , ahk_id %hMainEdit%
    SendMessage, EM_SCROLLCARET,,,, ahk_id %hMainEdit%
    Return

#If WinActive("ahk_id" hGUIMain)
F3:: GoSub GoSearch
^vk46:: GoSub SearchEdit   ;  "Ctrl+F"
#If WinActive("ahk_id" hGUIFind)
Enter:: GoSub GoSearch
#If

GuiEscape:
GuiClose:
    ExitApp

To send this message, call the SendMessage function as follows. 

lResult = SendMessage(      // returns LRESULT in lResult
   (HWND) hWndControl,      // handle to destination control
   (UINT) EM_SETSEL,        // message ID
   (WPARAM) wParam,         // = (WPARAM) () wParam;
   (LPARAM) lParam          // = (LPARAM) () lParam; );  

global FR_DOWN := 0x1, FR_MATCHCASE := 0x4, EM_SETSEL := 0xB1
ES_NOHIDESEL := 256
StringCaseSense, Locale

Loop 24
   text .= "Съешь же ещё этих мягких французских булок, да выпей чаю`r`n"

Gui, +hwndhGui
Gui, Add, Edit, hwndhEdit w400 h200 %ES_NOHIDESEL%, % text
PostMessage, EM_SETSEL, 0, 0,, ahk_id %hEdit%
Gui, Show
OnMessage(DllCall("RegisterWindowMessage", Str, "commdlg_FindReplace"), "FindTextDlg")
Return

GuiClose:
GuiEscape:
   ExitApp

#If WinActive("ahk_id" hGui)
^vk46::   ; Ctrl + F
   oFind := FindTextDlgInit(hGui)
   Return
   
F3::
   if oFind.hDlg
      Return
   
   if pDATA := oFind.pDATA
      flags := NumGet(pDATA + A_PtrSize*3, "UInt")
      , FindHandling(StrGet(NumGet(pDATA + A_PtrSize*4)), flags & FR_DOWN, flags & FR_MATCHCASE)
   else
      oFind := FindTextDlgInit(hGui)
   Return
   
FindTextDlgInit(hGui)
{
   static FINDREPLACE, FindWhat, FR_DOWN := 1, FR_HIDEWHOLEWORD := 0x10000

   if !FINDREPLACE  {
      FindWhatLen := A_IsUnicode ? 160 : 80
      VarSetCapacity(FINDREPLACE, SizeOfFR := A_PtrSize*10)
      VarSetCapacity(FindWhat, FindWhatLen)

      NumPut(SizeOfFR, FINDREPLACE, "UInt")
      NumPut(hGui, &FINDREPLACE + A_PtrSize)
      NumPut(FR_DOWN | FR_HIDEWHOLEWORD, &FINDREPLACE + A_PtrSize*3, "UInt")
      NumPut(&FindWhat, &FINDREPLACE + A_PtrSize*4)
      NumPut(FindWhatLen, &FINDREPLACE + A_PtrSize*6, "UShort")
   }
   Return {hDlg: DllCall("Comdlg32\FindText", Ptr, &FINDREPLACE, Ptr), pDATA: &FINDREPLACE}
}

FindTextDlg(wp, lp)
{
   global oFind
   static FR_FINDNEXT := 0x8, FR_DIALOGTERM := 0x40
   flags := NumGet(lp + A_PtrSize*3, "UInt")
   
   if flags & FR_FINDNEXT
      FindHandling(StrGet(NumGet(lp + A_PtrSize*4)), flags & FR_DOWN, flags & FR_MATCHCASE)
   
   if flags & FR_DIALOGTERM
      oFind.hDlg := 0
}

FindHandling(text, down, case)
{
   global hEdit, oFind
   static EM_GETSEL := 0xB0, EM_SCROLLCARET := 0xB7, MB_ICONINFORMATION := 0x40, MB_TASKMODAL := 0x2000
   
   WinExist("ahk_id" hEdit)
   ControlGetText, EditText
   SendMessage, EM_GETSEL
   start := ErrorLevel & 0xFFFF, end := ErrorLevel >> 16
   if ! pos := InStr(down ? EditText : SubStr(EditText, 1, start), text, case, down ? end + 1 : 0)
      Return DllCall("MessageBox", Ptr, (h := oFind.hDlg) ? h : hEdit
                                 , Str, "Не удаётся найти """ . text . """"
                                 , Str, "Find"
                                 , UInt, MB_ICONINFORMATION | MB_TASKMODAL)
   
   PostMessage, EM_SETSEL, pos - 1, pos - 1 + StrLen(text)
   PostMessage, EM_SCROLLCARET
}

#SingleInstance Force
#NoEnv
StringCaseSense Locale

ES_NOHIDESEL := 256  ; показывает выделение текста в Edit при неактивном окне
EM_SETSEL := 0x00B1  ; установка выделения
EM_SCROLLCARET := 0xB7  ; сделать видимым выделенный текст
EM_GETSEL := 0xB0  ; извлечь позицию каретки ввода
VarSetCapacity(start, 4), VarSetCapacity(end, 4)

Loop 12
    Text .= "Как можно сделать поиск в поле Edit?`n"

Gui, -DPIScale +HWNDhGUIMain
Gui, Add, Edit, x0 y0 w500 h100 vMainEdit hwndhMainEdit %ES_NOHIDESEL%, %Text%
PostMessage, EM_SETSEL, 0, 0, , ahk_id %hMainEdit%
Gui, Show, w500 h300, Edit find

Gui, 2: +Owner1 -MaximizeBox -MinimizeBox -DPIScale +HWNDhGUIFind
Gui, 2: Add, Text, ym+5, Что:
Gui, 2: Add, Edit, x+25 yp-5 w284 vFind hwndhFind, Можно
Gui, 2: Add, Button, x+10 yp gGoSearch, Найти далее
Gui, 2: Add, Button, xp y+5 wp gCancel, Отмена
Gui, 2: Add, Checkbox, vSens y+0 xm, С учетом регистра
Gui, 2: Add, GroupBox, x+10 yp-25 h50, Направление
Gui, 2: Add, Radio, xp+10 yp+25 vDirectUp, Вверх
Gui, 2: Add, Radio, x+15 Checked, Вниз 

SearchEdit: 
    Gui, 2: Show, , Найти
    Return
    
Cancel:
    Gui, 2: Hide
    Return

GoSearch:
    GuiControlGet, DirectUp, 2:
    GuiControlGet, Sens, 2:
    ControlGetText, MainEdit,, ahk_id %hMainEdit%
    ControlGetText, Find,, ahk_id %hFind%
    SendMessage, EM_GETSEL, &start, &end, , ahk_id %hMainEdit%
    If DirectUp
        f := (f := InStr(SubStr(MainEdit, 1, NumGet(start)), Find, Sens, 0)) ? f : InStr(MainEdit, Find, Sens, 0)
    Else
        f := (f := InStr(MainEdit, Find, Sens, NumGet(end)+1)) ? f : InStr(MainEdit, Find, Sens, 1)
    If !f
    {
        MsgBox, 8256, Edit find, Не удаётся найти "%Find%"
        Return
    }
    PostMessage, EM_SETSEL, f-1, f+StrLen(Find)-1, , ahk_id %hMainEdit%
    PostMessage, EM_SCROLLCARET,,,, ahk_id %hMainEdit%
    Return

#If WinActive("ahk_id" hGUIMain)
F3::
^vk46:: GoSub SearchEdit   ;  "Ctrl+F"
#If WinActive("ahk_id" hGUIFind)
Enter:: GoSub GoSearch
#If

GuiEscape:
GuiClose:
    ExitApp