The easiest way to invite new players to the game is by sending them a direct link.  This can be done by opening the invitation menu, pressing the 'Invite' button, and selecting your preferred method for sending the link.  No manual code is necessary, and the referral will be credited once the new player has completed Level 10.

The referral will only be credited if the invited player selects the direct link to download the game.  If it is downloaded from another source, they will need to input the Referral Code.