Add a custom submission handler to a form
If you use hook_form_node_form_alter() maybe use example code:
function MYMODULE_form_node_form_alter(&amp;$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  foreach (array_keys($form['actions']) as $action) {
    if ($action != 'preview' &amp;&amp; isset($form['actions'][$action]['#type']) &amp;&amp; $form['actions'][$action]['#type'] === 'submit') {
      $form['actions'][$action]['#submit'][] = 'mymodule_form_submit';
    }
  }
}

and for the submit function use:
function MYMODULE_form_submit(array $form, \Drupal\Core\Form\FormStateInterface $form_state){
  //die(&quot;I'm not getting run, why :(&quot;);
  \Drupal::service('messenger')-&gt;addMessage(&quot;Why won't this message show?&quot;);
}