Visit www.000webhost.com and login as you usually would do
Manage your website
Choose set web address
Browse down till you see your parked domain
Now hit manage and then link website
Choose your account you want to link
But now click "I want to use a sub-domain"
Now you can add a subdomain like forums.yoursite.com so your visitors can visit that and it will take them to your site easier.
Now it should return you back to the set web address page and the subdomain will have been added.