EXTRASEAT has worked for me by just booking it myself on united.com with the correct name for the extraseat (fake) passenger. I've done it using either EXTRASEAT or EXST as first name, with my same last name, and it worked both ways. I assume if TK books the extra seat that way, it would work also, but that's just speculation.
Thanks! Just to make sure I'm understanding correctly, in your case you're booking 2 tickets, on one reservation, from united.com. If your name is Ned Nickerson, then the first ticket you purchase for "NED NICKERSON" and the second ticket you purchase for "EXTRASEAT NICKERSON". Is that right?