I have a form with a timer on it but when I try to enable this timer from a prg, I'm always having error 1923.

In a prg you need to refer to _Screen.Activeform.SomeComtrol.

However, be aware that this may give you misleading results if there is an activeX control on the form. So you may need to do some more checking to make sure that the object on the form actaully exists by using the PEMSTATUS() function.