fill textbox on button click in MUI Directory Page

i have added custom button using Resource Hacker on the MUI Directory Page of the installer, now i want that when the button is clicked then the Default Textbox where the user enters the InstallDir (the path where the application is installed), is filled with some specified text. please help me with the code?

This is very simple, as I mentioned in this thread: http://forums.winamp.com/showthread.php?t=330068

Just use WM_SETTEXT to set text in Edit box:

!define DEFAULT_TEXT "C:\Program Files\Application"
Function CDRIVEPATH
FindWindow $0 "#32770" "" $HWNDPARENT
GetDlgItem $1 $0 1019
SendMessage $1 ${WM_SETTEXT} 0 "STR:${DEFAULT_TEXT}"
FunctionEnd 

Using ButtonEvent plugin, and you must make sure the button ID is the only one.