Use the OnObjectModify trigger on the tab control.
You can give each tab in the control an object name by selecting each tab in turn and entering the names in the name box at the top of the Inspector's position tab.
Then you can use GetLayoutObjectAttribute ("ObjectName" ; IsFrontPanel )
to determine which tab was selected by your mouse click.
Thank you very much Phil!
If ( GetLayoutObjectAttribute ("Invoice"; "IsFrontTabPanel"))
some script here...
and all worked :)
Thank you very much for your help.. AS ALWAYS!!