This guide is applicable to all users.
Even non-clanforge users! 

Once you've loaded the game into the main menu you simply need to hit F3 to open the console.

Then type (without brackets): Open [ip:port]

Clanforge Users:

You can find your IP & Port under the 'Address' section of your Server Dashboard.
It will look like this:

This is what the Command will look like as you type it: