You could say. "Hello. Would you mind if I add you as a friend?"

Make it short and sweet, straight to the point :) "I'd like to talk to you, please add me to your friend list!!"

Hello Jessie, Open lines can vary a lot, but should mostly start with some greetings of any kind. A reserved greeting, a short self introduction, then a direct request to exchange languages will do. Example: " Hello, how are you today . I am Jessie from China. I am learning English and I see you are learning Chinese . Would you be interested in a language exchange?" Important is you don't stick to certain wording , just be innovative and write what comes to your mind instantly.

Hello Jessie If you want to speak english just write "Hello How are you ?" I think it's enough to start a conversation isn't it ?