Get Custom UUID from User

I’m trying to get uuid from player that given in minecraft launcher startup by key “–uuid some_uuid”.
This uuid is getting from own website via REST api.He is generated in website. And I’m developing plugin, that should get uuid from player and compare with website. But, unfortunetly, when I’m trying to get UUID from player, this returns the generated by Sponge UUID.

public void onPlayerJoin(ClientConnectionEvent.Join event) {
Player user = event.getTargetEntity();
String uuid = user.getUniqueId().toString();


And my question, how to get the UUID from player, that given in minecraft launcher statup?

I feel like this might not be possible… Why wouldn’t you just use the proper Mojang auth system?

This is not possible. When logging in, the client only tells the server its version, the IP address (and port) it used to connect, and its username. The UUID is provided by the Mojang servers during the login process.