Using AEM 6.3 , I am trying to add a custom action type in AEM Form Core Component. It had only 4 dropdown items. I successfully added the custom action in the container dropdown.
For the post action, I followed the steps given in this post.https://helpx.adobe.com/experience-manager/using/form_component_customaction.html
But I am not getting the data in backend on submit.
Also - just so you know - the article that you referenced was for AEM Form that you create by using AEM Foundation components. If the form core component does not meet your needs - look at using the foundation form components.