How to authenticate users for an app which has a proprietary protocol for user communication

I have a server application, with which the client applications communicate with a proprietary protocol. Clients send the username and password after establishing a TCP connection to the server for authentication, and after the validation of the credentials, the TCP connection is assumed to be authenticated. It is not possible to do any change to the existing client applications.

I am currently managing user credentials in a database, which I am planing to migrate to Okta.

What is the best way I can achieve this? Currently I am thinking of using the authentication API at the server application to validate credentials, but I am not confident whether this is the best way.

Can anyone assist me to find the best solution for this?

I think you are a TS for this, so you can check the answer there