How likely is this to be true? Ridiculous, but so is everything else about the clown.
Obama was there.
Trump was there later.
Trump is plainly obsessed with Obama, and is horribly mean-spirited.
Russian prostitutes will often do whatever you pay them to do. Expensive hotels will clean up whatever mess you leave behind.
Trump has been accused of worse behavior.
The FSB (the new KGB, similar to the American CIA) routinely spies on and records whatever dirty little secrets that important people engage in behind closed doors, just in case some of it proves useful.
A British MI6 agent (basically James Bond) with a ridiculously good Russian information network (many of whom have since died) was asked to compile everything that was known about Candidate Trump. The report was meant for American intelligence, not the general public, but the report was leaked, and (among other information which the British agent relayed to America which has already been proven correct) he said that it is known within high levels of Russian intelligence that Putin has a videorecording of Trump's pee-party. Putin can use it to blackmail Trump, or just humiliate him out of spite.
Trump has inexplicably been bending over backwards to try and make Putin happy.
I'd say there's a 99.9% chance that the pee-pee tape is real.