I created a new pushbutton with function code "/*VA01",  in order to skip the initial screen of VA01. Since there are obligatory fields on this screen, I also created a GuiXT script that sets all necessary default values. But this screen still comes up with an error message asking me to fill in the fields.

With  "/*xxxx" the first screen of the transaction xxxx is not sent to the frontend, and GuiXT never gets the chance to fill in the defaults.Possible solution: you call the transaction with "/nxxx" together with an InputScript. In the InputScript you then set the defaults and skip the first screen with "Enter".