Redirect domainA/forum to domainB/forum

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I use to have my forum on domainA/forum so a lot of people still access it via they history to that site.
Now I have redirect in .html so when they go to domainA, they will be redirected to domainB.
I still couldn't find a way so when they go to the forum on domainA, it will redirect to forum on domainB

Here is what I have but it doesn't seem to work

RewriteCond %{HTTP_HOST} ^domainA.com/forum/$ [OR]
RewriteCond %{HTTP_HOST} ^www.domainA.com/forum/$
RewriteRule ^(.*)$ "http\:\/\/www\.domainB\.com\/$1" [R=301,L]