Authorization OAUTH2, unknown receiving server port for access token

[IndyBeginner]

Hi! I am working on project to send mails using SMTP server and OAUTH2
autorization mechanism with Indy components. There is a moment, when I 
authorizate and it is necessery to open http server to receive the
access token. Before I open the server, it is necessery to know the
default, free, port. It is possible in some situations, that the
previously chosen port can be unavailable in this moment. I am
wondering, if is it possible to set that the server will choose the
first free port or there is no way to do that and I need to know that
the port I set is free? I am the beginner. Thank You for reply.