Hey, I hear my name! 
But yeh, with a bit of a hacky solution it's actually possible:
https://bitcointalk.org/index.php?action=pm;sa=send2;to=NLNico;bcc=NLNico;message=test%0Anewline;subject=test%20subject
- You can use either "to" (username) or "u" (id)
- As example I included "bcc" and "subject" too, obviously you can change/remove that
- Use "%0A" for newline
- Use "%20" for space
- It is hacky because it actually links to the "second send page", but since there is a protection (so you cannot trick people into sending a message), you get a "session expired" error. But it technically works for your purpose
But yeh, with a bit of a hacky solution it's actually possible:
https://bitcointalk.org/index.php?action=pm;sa=send2;to=NLNico;bcc=NLNico;message=test%0Anewline;subject=test%20subject
Code:
https://bitcointalk.org/index.php?action=pm;sa=send2;to=NLNico;bcc=NLNico;message=test%0Anewline;subject=test%20subject
- You can use either "to" (username) or "u" (id)
- As example I included "bcc" and "subject" too, obviously you can change/remove that
- Use "%0A" for newline
- Use "%20" for space
- It is hacky because it actually links to the "second send page", but since there is a protection (so you cannot trick people into sending a message), you get a "session expired" error. But it technically works for your purpose
I forgot about this thread so I didn't see your post
but thank you so much, this will come in handy!! 
