Executing Mask parameter callback only when "Apply" is pressed

I have a masked subsystem. the mask dialog contains a Popup element. The popup elements has values from (1..5). This value indicates the index of an array in the workspace. Then i have an edit field. The value of the edit field should be written to the index of the workspace variable which is indicated by the popup element. I implement a simple callback for the popup element which looks like:

Nr = str2num(get_param(gcb,'PopupNr')); MyValue(Nr).a = str2num(get_param(gcb,'a'));

but i want this callback to be executed only when I press "Apply". How can I do this? (remember.. there are no stupid questions)

Thank you