Help with userexit_save_document

I create a new tab in sales order head information called addational data .and i put a component named 'add_address' . My problem is when I click the save button. the 'add_address' will be saved default value when it is initial. otherwise, it will be saved with input value.

Now what I do is :

Form userexit_save_document.

if xvbak-zz_address is initial.

xvbak-zz_address = 'defalut address'.

endif.

endform

but it didn't work...

anyone can help me?