How to trigger Mailster autoresponder when an existing subscriber subscribes again via action hook?

Question

I am setting up an autoresponder from Mailster which will get triggered when an existing user signs up to my Newsletter again. I can't seem to figure out to trigger the autoresponder. This is the guide I am following: https://kb.mailster.co/working-with-action-hook-auto-responders/

I have tried template redirect as when user signs up to my newsletter, they are getting to redirect page to download the font. But it is still not working!

My website is: https://squarekufic.org/

Mailster is a newsletter plugin for Wordpress. For the function code, I am using Snippet.

Your help is really appreciated.

<?php 
////////////////////////
add_action('template_redirect', function() {

    $subscriber_id = mailster_get_current_user_id();
    if (is_page(79) && $subscriber_id = mailster_get_current_user_id() ) {
        do_action( "resend_the_email", $subscriber_id );
    }

} );