Simple Task. Use freelancer api.
Give information/web links on how to register for api keys.
Use any server/language you like, for example, iis/[url removed, login to view], c# or apache, java, ruby or whatever.
Create a form on a web page with all the required fields from the api to post a job.
When clicked post, post it to the server you created. Then let server post it to freelancer.com. After successful post, display the posted job's url on the web page.