title: Add custom field to wordpress plugin
body: <p>I want to add custom field to this plugin
  <a href="https://wordpress.org/support/plugin/wp-abstracts-manuscripts-manager/" rel="nofollow noreferrer">https://wordpress.org/support/plugin/wp-abstracts-manuscripts-manager/</a>
  Add field to events tab</p>
  <p>Do anyone has an idea?
  I have tested with many solutions but non of them has worked :</p>
  <p>mu-plugins add custom-function.php and add</p>
  <pre><code>if(!function_exists('function_name')){
  function  function_name(){
  }
  }
  </code></pre>
  <p>there is no filter in the plugin that make the process easy , the plugin is not overrided by copying it to theme directory
  thanks in advance,</p>