You don't. SMART checks TMBR to see if your order is marketable against its bid or offer (always at or better than the NBBO). If it is, that's where you execute. Otherwise, it goes wherever it would go if there were no TMBR (i.e. NYSE or AMEX for listed stocks).
I've been trading with IB since May of this year, and I must say that IB smart order routing rocks. I am getting the best fills I have ever gotten.
My bad - you can't set TMBR as a default globally but you CAN specify it on the order entry line(s). That's where I set it.
that's the funniest thing i've read all day... my special NYSE friend was a real d!ck today. must need to get his quota for the month.