Php İyzico samesite session how to fix it?

I am use iyzipay . mydomain.com forward to iyzipay.com for payment. Payment finish and callback url mydomain.com/paymentresult.php, but mydomain.com session is killed and paymentresult.php forward to login .php page. => this samesite problem . How to fix it ?

Current PHP version: 7.2.34

My Code in paymentresult.php

   if(!isset($_SESSION['username']))
    {
    header("Location:login.php");
    }
    else
    {
    payment result jobs
    }