Hi Koning,
Here is solution for you:
1 Open your "zoeken" service on "request" tab.
2 Move "bondsnummer" parameter from "query string" to "body". http://prntscr.com/4rxc04/direct
3 Navigate to your service mapping and create link from "bondsnummer" component to "bondsnummer" body parameter. http://prntscr.com/4rxcfp/direct
That's all.
Regards.