I have asked you if you managed IIS part of this task and you didn't mentioned anything about it, so I supposed you did solve that part. Ok, now you need a sample on how to do this, well here
it is. This is a simple way of solving your task but note that this isn't related to the MonoX.
Basically it comes down to this:
Setting up DNS
If your DNS supports wildcard syntax, you’ll be able to do this. Basically you just want to forward all requests to *.yourdomain.com to yourdomain.com. If your ISP’s dns doesn’t support the wildcard syntax, you could handle your own DNS on your server.
some IIS configurations and a custom HttpModule, please read the article and get back to us if you need anything else.