Once Bull’s plot is complete and you have won Bull’s approval, you get a scene reacting to the events of the plot, with Bull either reminiscing about the Chargers or dealing with Ben-Hassrath assassins.
After this conversation is complete, if you have flirted with Bull on previous occasions, then the next time you speak with Bull, he will show up in your room to talk with you. If you are interested, confirm that you want to have sex. When that scene is done, speak with Bull again and ask to talk about what happened, and Bull will discuss the rules of your relationship, if you choose to pursue one. If you’re interested, you can now begin a relationship with him.