How do I prefill a contact form with comments?

I have at least three contact form plugins installed (jetpack, XYZ, Unipress…) but I can’t for the life of me figure out how to prefill the comments part of the form. I run a campaign website and wish to have a prefilled letter to be sent to our county council, all I want users to be able to do is enter their name and e-mail and click send and a letter will go to the required address – anyone know how to do this with any of the plugins?