It used to be in the addendum but now in the new (100th revision) of the listing agreement, there is a place for "commission" or "concession" and what you should indicate should be something like:

"Seller to pay ____ (hopefully 4%) to buyer's agent upon successful providing buyer and offer acceptance contingent on close of escrow."

If your listing agreement does not have that, then just add something in the notes section of the contract that says seller to pay____ (hopefully 4%) to buyer's agent.

BUT...we always recommend checking with your Broker, as they have the final say.

Good Luck and Good Selling!