Get-Parameter causes 404

Say I have my form on the URL http://blog.vfmb.ch/ticketverlosung-glarus-sud-trifft-sich/, how can i pass the get parameters for pre-populating form fields? Adding them behind a question mark just causes a 404 error.

Thanks for your help
Benjamin

http://wordpress.org/extend/plugins/contact-form-7-dynamic-text-extension/