use muti-city, then hold the reservation, don't ticket it!

call united.com CSR, not UA reservation, tell them that this is a round trip flight, but the web cannot recongnize, so you cannot use the certificate online. then, they will ticket for you and tell you they will ask Customer Relation department(something like that ) to look at it, if it is a roundtrip, they will refund the difference.

Also, they took the coupon number!

Next day, I get another email with adjusted price!

In addition to that, you will earn online booking bonus, but you just have to call after you finish the flight!