Hi Jari,
I've played around with it a bit, and I think it's probably do-able - but I also think you're on completely the wrong track. Embedding that sort of code in a form - and especially in that position - is not "correct" and in the long run will probably break (and break other things.) You're thinking of the form in a very "windows" way, and the form doesn't work like that.
I think it's probably best to go back to the root problem you are wanting to solve - which is the user selecting a date. There's a "Date" Form field type for that - and that field has lots (and lots) of options - many of which you are probably not aware of. so the first thing I'd probably recommend is that you construct your form using a date field - and then perhaps we can discuss how you'd like the date field to behave vs how it behaves on your form.
So, in short, I'm not sure I'd use the NetWebYear as the starting point here. That's primarily for displaying data - the Date Form field type is primarily for capturing a date from the user.
(I understand it's not going to work like you have in mind, but let's start there and discuss it from there.)
cheers
Bruce