I am doing taxes for my daughter who is also a first time owner of a co-op. I have owned several houses and a co-op but ran into the identical issue when inputting into Turbotax. The managing agent of the complex did not supply the beginning and ending underlying mortgage balances for 2018 so I reached out and they will be supplying them. From there, the shares from the unit in question divided by the total shares in the complex provides her percentage of the underlying mortgage (about 1%) that I will apply against the total mortgage number they give me (for the months she had the co-op during 2018). I don't see any other logical way to input her underlying mort interest paid. BTW, Turbotax help line was absolutely useless.
