The advantage of this method is that you don't have to moderate who wants to access the forum, and can change the password at any time to lock everyone out.
If you find the constant password prompting objectionably inconvenient, you may prefer to use a group. When using a group, users can request to join the group, and have that request approved by a moderator. If the forum has been restricted to that group, users will then be automatically permitted access to the forum unless they are removed from the group. The catch is that a moderator is required to approve all the join requests. You must be the moderator with password-setting privileges in a forum or an administrator to set a forum password. The forum must also have been set to allow setting of a password.
Log into the moderator interface at https://forum.cs.umd.edu/modcp.
Click on Forum Manager on the left menu bar.
You will see a list of all forums. Those for which you may set a password will show an Edit Password link. Click it.
Enter the password for your forum and click Save.
Do not apply this password change to child forums unless you know what you're doing. In most cases, your forum will have no child forums and this won't do anything anyway.