Close process

Okay, you were right, closewindow only minimizes the window. So again, a new attempt:
I've added some delay and safety so the computer doesn't hang when nothing is found:

ExecFile "Calc.exe"

windowname$ = Input("Please enter window title >")

find = 0
While find = 0
	find = api_FindWindow%(0, windowname$)
	Delay 200
	c = c + 1
	Write "."
	If c > 15 Then Exit
Wend

If find = 0 Then Print "Window was not found..": WaitKey(): End

Write "found:" + find

api_PostMessage(find, $10, 0, 0)

RuntimeError "Ok!"