If you're going this route, then it's best to contact the administration, and if they give the approval, then don't just join to promote. Try hanging out with their community for a few months, contributing your own thoughts in writing. Show their community that you are a reliable person who's worth his salt. Once they like you enough to want to check out your forum in your signature, then you know you've made a companionship.
Imagine a competitor joining YOUR forum to advertise. How would you and your community feel? Also, messaging members isn't the best route to take. They have no idea who you are, and they may feel even more turned off from ever joining in the first place.
Don't advertise the platform, advertise the content (where relevant). Not only does this see better results in general practice, but in this case makes it hard for someone to justify blocking your advertisement, since it contributes to an existing discussion.