Here ya go!
Linking to another site
phpBB BBCode supports a number of ways of creating URIs, Uniform Resource Indicators better known as URLs.
The first of these uses the tag, whatever you type after the = sign will cause the contents of that tag to act as a URL. For example to link to phpBB.com you could use:
Visit phpBB!
This would generate the following link, Visit phpBB! You will notice the link opens in a new window so the user can continue browsing the forums if they wish.