To add a referral to a licensed agent/broker they must first be entered as a contact via the people menu as shown below:
If the referral is a licensed party (Broker or Agent) they are added as an outside Brokerage profile type. If the referral is to an unlicensed person or organization please see the article related to adding third party referrals.
Once the party has been added you are now able to use them in the allocations screen by creating a split as shown below.
Be careful to ensure that when the Split for any referral is made the 'End' box reflects what is happening on the Deal. For a Referral the 'End' box would be set at zero.