Passing a field value via a query string

Hi

I'm having a problem passing a value from a drop down to a query string on a "thank you" page.

I have a drop down with an admin field label 'gtwid'. Each choice has a choice label that displays in the form - 'Webinar 1' 'Webinar 2 (free)' etc. Each choice has a value attached to it, (e.g. '54321') which I want to pass to the thank you url via a query string, but the choice label is being passed instead, see below;

http://www.domain.co.uk/2012/09/10/thank-you-for-your-webinar-booking?firstname=Fred&lastname=Bloggs&email=fredblogs%40domain.com&gtwid=Webinar+2+%28free%29&ServerNum=3

How do I pass the value rather than the label via the query string?

Thanks

That's excellent that is just what I wanted to do. Now then, is it possible to get 3 merge tag values from a date field e.g something like {day} {month} {year} ?