Use SendMessage EM_SETSEL. Example:
; .lib "user32.dll"
; SendMessage%(hwnd%, wMsg%, wParam%, lParam%):"SendMessageA"
Const EM_SETSEL = 177
wndMain=CreateWindow("",200,200,200,200)
tarMain=CreateTextArea(0,0,100,100,wndMain)
lblMain=CreateLabel("Cursor pos: ",0,105,100,20,wndMain)
btnMain=CreateButton("Move Cursor",105,0,80,20,wndMain)
SetTextAreaText(tarMain, "Text Area Text Area Text")
Repeat
Select WaitEvent()
Case $803 : End
Case $401
Select EventSource()
Case tarMain
SetGadgetText(lblMain, "Cursor pos: " + TextAreaCursor(tarMain))
Case btnMain
SendMessage(QueryObject(tarMain,1), EM_SETSEL, 1, 1)
ActivateGadget(tarMain)
SetGadgetText(lblMain, "Cursor pos: " + TextAreaCursor(tarMain))
End Select
End Select
Forever