>Actually, it seems to be something like navigate('REZ://...

No, it is this

<FORM METHOD="post" ACTION="rez://ADDITEM" ID="frmAddItem" NAME="frmAddItem">

and the button is of type submit.

So, somehow all the form's values are added to the string being posted. May be there is a different way. Say, instead of the action somehow send a hidden input? E.g. I can add some extra functionality to set some extra hidden input to be btnRemoveOverdue. My code just checks for the 'btnRemoveOverdue' to be inside the string.

I'll try this idea now instead of altering the action.