Plugin to add the OptinMonster After Post template tag.

om-add-after-post-optin.php

<?php

/**
* Plugin Name: OptinMonster Add After Post Optin
* Description: Add the 'd1ztn1euuw-post' After Post optin to single blog posts.
* Version: 1.0.0
*/

add_filter( 'the_content', 'ekf_add_om_after_post_optin', 100 );
function ekf_add_om_after_post_optin() {

    if( is_single() ) {
         optin_monster_tag( 'd1ztn1euuw-post', false );
    }

}