create-license.php

function wpi_s2_payment_notification() {

    if ( !empty( $_GET['yourauthenticationtoken']) && 'yes' === $_GET['yourauthenticationtoken'] ) {
        // in the URL, I have '?yourauthenticationtoken=yes', that's what I'm looking for here.
        
        // make sure the request comes from a valid user
        if ( !empty( $_GET['user_id'] ) ) {

            // Load the user data for the new license
            $user_id    = (integer) esc_attr( $_GET['user_id'] );
            $user       = new WP_User($user_id);
            $first_name = $user->first_name;
            $last_name  = $user->last_name;
            $email      = $user->user_email;
            $username   = $user->user_login;

            // generate the request to create a license key
            $api_params = array(
                'slm_action'    => 'slm_create_new',
                'secret_key'    => 'yoursecretkey',
                'first_name'    => $first_name,
                'last_name'     => $last_name,
                'email'         => $email,
                'max_allowed_domains'   => 1,
                'date_created'  => date("Y-m-d")
            );

            // send query to license manager server
            $response = wp_remote_get(
                add_query_arg(
                    $api_params,
                    'https://wp.immo'
                ),
                array(
                    'timeout'   => 20,
                    'ssl_verify'    => false
                )
            );

            // Check for error in the response
            if (is_wp_error($response)){
                echo "Unexpected Error! The query returned with an error.";
            }

            // License data.
            $license_data = json_decode(wp_remote_retrieve_body($response));

        }

        exit;

    }
}