UM flights are restricted to certain flights of the day, this is why you cannot do it online. You need a certain connection time and there needs to be at least another flight out that day if there is a connection. UM's are the last off the plane and then taken to their gate or UM lounge. There is a significant fee for this service (each way). There is no escaping it, my kids fiercely grumble about it. They have gone to wristbands instead of kindergarten dog tags of shame they used to have. Even if you're a few days shy of your 15th bday you have to do the UM service.