I know you are not microsoft support here. But will the setup I followed work for shared mailboxes for support agents since they are authorized by the main mailbox that I have set up on OSTicket?
For example, my main account is support@somedomain.com and is what I used to setup oauth2
Then I have a shared mailbox called agent@somedoamin.com that is tied to an agent under osticket. The "support" primary mailbox has a full owner and send as and read rights to the "agent" shared mailbox.
I do this because osticket will not let me have an agent for tickets that is the same email as the admin account and system account. So I use the agent email account for getting new tickets and replies.