Update UDF in different tab

Hello guys,

I have a small problems that I like to solve.

I want to copy the value of a field(Pane 0) into a different field. The second field is located in a different tab and is not visible at the start. The value is being copied if I make the tab visible.

Here is my problem. If I don't click on this tab and try to copy the value I get an Error: Item - Form item is not editable.

Here is my code which works perfectly if the tab is active:

Regards

Kai