For your character to learn JSL you'll need to submit an application
here.
Some tips for the application though that will help you get approved for your character to know the language and for you to get it in game that I have found helpful at least when applying for languages.
- How did your character learn JSL? Explain different techniques such as flashcards, online courses, in person practice and use, how often they practiced, etc. your character used to learn this language.
- How did people around your character react to your character learning? Did anyone help them learn such a parents, friends, guardians, etc. Who practiced with them if at all?
- Why did your character learn this language? Are they deaf? Mute? Did they learn it to speak with someone they knew that used JSL to communicate?
- How long did it take for them to learn this language? Characters or even people irl don't learn a language overnight, it takes years of practicing, learning, following example, etc to become any sort of fluent in the language. For example, when I was learning ASL IRL, it took me 2 years before I was able to talk in ASL in any sort of full sentence but even then I didn't know many sentences/signs.