What’s the deal with a joke at the Trump rally that was too offensive to repeat?

I read online that while there was an offensively racist joke about Puerto Rico, there was also a “cum inside” joke that no news outlet wants to even talk about because it’s too offensive to repeat. Some say it’s “sanewashing” to ignore the joke. So… What was that joke?

A source: https://www.reddit.com/r/NPR/s/1HRvLKvYsf