How to Include "onclick" Object in WordPress HTML

It appears that with current Wordpress (I'm on 4.9.4), TinyMCE does the filtering directly on the editor screen, not when the form is submitted. The allowedtags and allowedposttags don't seem to matter, so the solution above does not solve the problem for me.

The method I have developed uses the tiny_mce_before_init filter to alter the allowed tags within TinyMCE. The trick is to add the extended_valid_elements setting with the updated versions of the elements allowed for a.

First, look in the page http://archive.tinymce.com/wiki.php/Configuration3x:valid_elements to find the current value for a, which right now is

a[rel|rev|charset|hreflang|tabindex|accesskey|type|name|href|target|title|class|onfocus|onblur]

And add to the end of that the onclick attribute:

a[rel|rev|charset|hreflang|tabindex|accesskey|type|name|href|target|title|class|onfocus|onblur|onclick]

Then use that in the filter function like this:

function allow_button_onclick_mce($settings) {
  $settings['extended_valid_elements'] =  "a[rel|rev|charset|hreflang|tabindex|accesskey|type|name|href|target|title|class|onfocus|onblur|onclick]";
  return $settings;
}
add_filter('tiny_mce_before_init', 'allow_button_onclick_mce');

which you install in your functions.php file in Wordpress. You can see it in action by toggling the text and visual view on the edit page. Without the extended list, the onclick goes away. With it, it remains.

You can solve this by changing the anchor tag into button and adding a script. For more info please refer to this link: Wordpress TinyMCE Strips OnClick & OnChange (need jQuery).

By resolving, I'm assuming you mean to allow the onclick attribute. You will want to be careful with this, because modifying the allowed tags does this for all your users.

You can modify the list of allowed tags and attributes, by adding this to your functions.php file:

function allow_onclick_content() {
  global $allowedposttags, $allowedtags;
  $newattribute = "onclick";

  $allowedposttags["a"][$newattribute] = true;
  $allowedtags["a"][$newattribute] = true; //unnecessary?
}
add_action( 'init', 'allow_onclick_content' );

I suggest trying it with only $allowedposttags first to see if that works for you. According to this other stackexchange post, you should only need allowedtags if you need it for comments or possibly non-logged-in users, but when I did something similar in the past, I needed both of them to work.

On a side note, if you want a list of all already allowed tags and attributes, look inside your /wp-includes/kses.php file.