Member account validation (REST API)

[Mondain]

Hello

I'm developing a small service and the users can write comments. The users should logg in to write a comment, but I will prevent them to register a new account, because they are already registred in the IP Board.

The user simply login with IP Board username/email and password. I validate it and set a $_SESSION['logged_in'] variable to true. Very simple.

But I don't understand, how I do this. I guess it is not possible with REST API (I was very confused as I realized that, because in my opinion, it should be possible).

In the past, I used this script with IPB 3, but it is not working with IPB 4.
https://pastebin.com/t0vtY2Pk

Can somebody help me?

PS: I already start a search, but I couldn't find a easy way.