The structure of your link would look like this:
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https://hackforums.net/private.php?action=send&uid=UID&subject=SUBJECT&message=MESSAGE
Replace UID with the recipient's UID, the SUBJECT with the subject you want, and the MESSAGE with the message you want.
You don't have to include the subject or message parts. For just adding a message do this:
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https://hackforums.net/private.php?action=send&uid=UID&message=MESSAGE
and just to add a subject do this:
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https://hackforums.net/private.php?action=send&uid=UID&subject=SUBJECT
Then just use URL tags as you usually would as such:
Click To Message Emy
the code for which would look like this:
Code
[url=https://hackforums.net/private.php?action=send&uid=956054&subject=Hey Emy!&message=This is an example message link]Click To Message Emy[/url]