[terminallydoomed]

I've just purchased my first website and I'm wanting to redirect a subdomain to my invisionfree forums. I figured there are probably plenty of code whizzes here who would know how to do this sort of thing.
Now, I've managed to redirect by putting a .htaccess file with a redirect command in the subdomains http folder, but when it redirects it comes up with the invisionfree address whereas I want it to remain masked as the subdomain, to make it look more professional.
What I mean is this:

A - forums.blahblah.com
B - invisionfree.com/blahblah

I'm redirecting A to B, but want the address bar to keep A in it, rather than changing to B. How do I go about doing this?
Cheers in advance.