It looks like the site doesn't take coterminal airports, like NYC and WAS. I'm trying to book IAD-EWR + EWR-DCA, mainly due to the available award timings for each. Yet it won't take. The website forces a 3rd DCA-IAD leg, which won't have any award space (nor any flights at all). I'll have to call/email them? I'm trying to see if I can pay the $25 just once, rather than separately booking the two flights, at which point I might just be better off using cash or Krisflyer.
Edit. I figured it out: In Star Alliance mode, it forces the 3rd line to get filled out. But if you set the airline to United, it keeps the 3rd line blank. I was able to finish querying and ticket everything: