Select dropdown form by URL

I feel kind of dumb asking this question… I think the sollution could be quite simple.

On this page: http://ift.tt/1IZoRfk there's a form. And on the subpage I'm linking to the form. What I want to do is, choosing one of the «Betreff»-Select-Dropbdown by URL. Is this possible anyhow? I hope you can understand, what I mean… ;-)

Thanks a lot in adavance for your help!