How do you wear a knotted racerback tank?

A bra would be really tacky. I would say get a strapless bra or a stick on bra. I wouldn't recommend the stick-on ones that have sticky over the nipple (it really hurts to take off), but they have some that just have sticky pads attached to the cups. Other than that, you could try a razor back bra made for these types of tops, but they are generally pretty expensive and hard to find. Good luck!