How do I get session storage across subdomains?

Question

So I have two subdomains translate.mysite.com and www.mysite,com and on the translation it makes sure you have a certain username to access it but, if i sign it with the right account it always redirects

<?php
include("https://api.tylernodes.ml/auth_session.php");
if ($_SESSION['username'] == "samplename1" || $_SESSION['username'] == "samplename2") {
  echo '';
} else {
    header("Location: https://www.tylernodes.ml/");
}
?>