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 m 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".