Say I have several categories in my Drupal contact form, and I'd like to create a link to the contact form, but with one of those categories preselected, and, while I'm at, prepopulate the subject and message fields, here's how I go about it.
(1) install and enable the prepopulate module.
(2) Figure out which category (recipient) you want to preselect by finding its ID on the page example.com/admin/structure/contact
(3) Say your message is "this is my message" and your subject is "I'd like some more info & pricing please": pass those two strings in php's urlencode() function.
(4) Put everything in the following format:
Note that if you want several lines on your message, a newline is encoded as %0A (for some reason I can't get urlencode() to generate this for me)