Really? I thought this was technically possible but in reality nearly no agent is trained well enough to know where it goes.
In fact, getting the agent to sell a ticket is sometimes a hassle, nevermind getting the agent to realize how to select the lowest fare or know where to put your Free Spirit number in.

Would be happy to hear success stories. But also, what box or field to put the code in (in case we need to guide an inexperienced agent).