luceos , thanks for quick reply.
Actually, I like the Authorization code flow, meaning, whenever a member access to my app, it has first to authenticate with Oauth2 as he will log to Flarum site, once he enters his credentials (user/password), he will be presented with a consent page to continue, by accepting Oauth2 will send back a code which my api will use to ask for a token. I like this pattern since it's more secure to me but it has to deal with configuration with redirect uri and make the whole works in concert.
Thus, I don't need to persist anything on my end and members from Flarum can use their credentials to access to my app.
It will save less time to code since I already have in place the mechanism done with other providers like discord, reddit, ips, xf or the like.
But last resort, I can use rest API to do that too, just wonder if Flarum has something built in I can use.
Not sure for Laravel Passport, I have to do some reading as Flarum is totally new to me.
/LR