The filter takes an associative array of external plugins for TinyMCE in the form 'plugin_name' => 'url'. The url should be absolute, and should include the js filename to be loaded. For example: 'myplugin' => '<a href="http://mysite.com/wp-content/plugins/myfolder/mce_plugin.js">http://mysite.com/wp-content/plugins/myfolder/mce_plugin.js</a>'. If the external plugin adds a button, it should be added with one of the 'mce_buttons' filters.
Parameters
$external_pluginsarray
An array of external TinyMCE plugins.
$editor_idstring
Unique editor identifier, e.g. 'content'. Accepts 'classic-block' when called from block editor's Classic block.