By default you may send a pm via a user's forum profile page (e.g. see link 1). With this script you may also send a pm via user's Greasyfork profile page (e.g. see link 2).
Forum profile: https://greasyfork.org/en/forum/profile/1/JasonBarnabe
Greasyfork profile: https://greasyfork.org/en/users/1-jasonbarnabe
the button doesn't appear in your own Greasyfork profile page, and
it only works when you are logged in, for obvious reasons. Also,
it now works even without Citrus GFork.
Update 11/12/2016: The script was not working ok (because you have to simulate a keypress sequence -with a delay between each press- in order the username to be "typed" in the autocomplete popup, and so the script to be able to click it, to select it). I fixed it, but, in order to manage sending this simulated keypress sequence ( 1-line in the script! - line 47) I have to use the following require's :