Facebook messenger account linking
The account linking feature does not support this type of token validation. You would need to send a request to your auth server to check if the person is still logged in.
I was able to get the callback with the redirect_uri and auth code and I was able to authorize the user and redirect him but I am not getting the account_linking in the request object after successful login ie.., I want to check whether the user is logged in or not for every message he sends.