If you’re trying to access the environment variable when tunnelling, make sure you prefix the variable with FORGE_USER_VAR_, but otherwise you’ll need to set the environment variable forge variables set key value and then re-deploy in order for it to take affect. Note that variables set this way are only accessible directly from your Forge backend. If you want to access the variable from your custom UI, then you’ll need to call your backend to get the variable.
Regards,
Dugald