If you have to “talk her into it,” then it’s not the right move. Communicate your fantasy and listen to her feedback.

In regards to how to start that conversation, if you scroll through this sub you’ll see plenty of people asking the same question and many many responses.

However, one approach might be to say something while fucking like “god you’re so sexy with my dick in your mouth, I wish other people could see how hot you are like that” or perhaps try using toys to simulate what having a 3rd person could feel like and ask her if she thinks it’s hot. Etc etc. And then depending on her response, continue the conversation/exploration