Facebook Messenger: get access token of linked acc

Question

First off, I'm not looking for the access token of a facebook account.

I (or my facebook messenger bot) need the access token of a linked 3rd party account.
The account was successfully linked as described here: https://developers.facebook.com/docs/messenger-platform/identity/account-linking

Slightly off topic: The first issue I noticed that does not meet the OIDC/OAuth2 specs is, that the redirect URI given to the /authorize endpoint expects a param "authorization_code" instead of "code" with the actual authorization code. Ok, good thing we do not use off-the-shelf software for the IdP, so I could mend that. Yet.

Now, I'm accustomed from the Amazon Alexa account linking to let the amazon do the token management, refreshing etc. and to deliver an 3rd party access token with every request.

But not with Facebook, it seems. As far as I managed to understand (not by the docs, but by try&error), for the Facebook Messenger the account linking is finished once they get the authorization code (and call an associated webhook). But since I need an actual access token for every request to my bot, that's not gonna help much.

It now looks as if I have to extend my bot to be a complete, but not standardised, OAauth2/OIDC client (including token handling, refreshing etc.) with some problems:

• how to check the validity of the redirect URI?
  • should be the same as in the initial request
• how to validate the authorization_code?
  • keep in mind, normally this bot has no connections/access to the database of the IdP
• how to link this to a specific facebook user id?

My question now is, how to cope with that behaviour when I aim to use standardised software for the OAuth/OIDC IdP? Any ideas?

Thanks in advance Thomas

EDIT: To clarify what is meant by "account linking": I'm not interested in the users facebook account, I want to access information of a 3rd party account (provided with my actual service) of that user. For example a call like "How many pictures are in my tumblr account" will need the customer to link the tumblr account to this facebook messenger bot.

For comparison: this worked extremely smooth with Amazon's Alexa and Google Actions. I simply had to input the client credentials and OAuth endpoints and thats that. If the user successfully linked the 3rd party account, I get an access token with every query to my backend, as expected.

EDIT2: as pointed out by CBroe, this is not supported by Facebook. Only actually "linking" the User IDs is possible.