Welcome!
I’m not a member of the Mozilla team, but I wanted to tell you that there’s currently a delay of 2 weeks or so for this sort of task.
The ISO 639-3 code for Shan is shn (639-3 is the name of the standard for all languages). I assume the script you will use is the Myanmar script, code Mymr on the list linked from the post 📖 Readme: How to see my language on Common Voice.
Craig