You can derive the add on ID based on where the addon.json file is
If it’s in src/addons/XFMG/addon.json then the add on ID is XFMG.
If it’s in a “vendor prefixed” directory like src/addons/XR/PM/addon.json then the add on ID is XR/PM.
Another way is via the Admin CP. Any URLs related to add on actions such as upgrade or install will contain the add-on ID.
If you see a dash in the add on ID portion, replace that with a slash. e.g. if the add on URL is XR-PM then the add on ID is XR/PM.