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.
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.
Facebook messenger account linking