Hi Daniele,
As you can't write directly to the datasources on a system form, I typically use the following steps to change a value on a non-active tab:
1) Read the current pane level and store in a variable.
2) Freeze the form.
3) Call the click method of the folder object where the field I want to change is.
4) Change the value on the field using the item properties or methods.
5) Click on the folder object for the original pane level.
6) Unfreeze the form.
Depending on the form design, you might have to call the click method on a field on the original pane (or on a field that is available on all pane levels).
Kind Regards,
Owen