Probably not... A lot of forums these days have firewall scripts and other security measures to keep members from injecting code into the forum so more than likely, that could be your problem.
If so, then as barsha says, you may want to invest in some urls simply for redirecting to your affiliate link... If you purchase a .info domain, this can be a pretty cheap way of redirecting to your affiliate link.
In a lot of cases, I've been able to obtain the exact same domain name of the affiliate product but with a .info instead.
But I also use .net .biz and .org...
And if that domain is not available with any of the above extensions, then I simply play with it like this...
Let's say the domain name is:
perfectpoison.com
You could purchase:
purfectpoison.com
perfectpoison.net
perfectpoison.info
perfectpoison.biz
perfectpoison.org
But none of those are available so you play a little more:
getperfectpoison.com
perfectpoisonnow.com
myperfectpoison.com
theperfectpoison.com
Get it? Then simply login to your registrar and redirect the domain to your affiliate link. Now use your new domain on the forum.
filmy