Ahh, that makes more sense... having a lower priority (a higher number) means that it will run after your code rather than before... which is what was previously causing your code to basically overwrite what I had defined. 

Thanks 😀 

The logic you're using is correct. You've found the right way. 

However, you've done some customizations in the class-template.php file, that will be lost after the wpForo update. In case, if you don't want to make the same changes each time after the plugin update, just do it via the hook.

So the correct way (without modifying any wpForo core files) to get it resolved is calling the "mce_external_plugins" filter and use the priority greater than 15 (for example we use 16 in the example below 16).

For adding the buttons in the TinyMCE toolbar you've written the correct way in the post above (using the "wpforo_editor_settings" filter). 

Finally, to add custom TinyMCE plugin to wpForo editor you just need to write the small functions as shown below

//to add custum plugin 
add_filter( 'mce_external_plugins', function ( $plugin_array ) { 
        $plugin_array = plugins_url() . '/crayon-syntax-highlighter/util/tag-editor/crayon_tinymce.js'; 
        return $plugin_array; 
} , 16 );


//to add the buttons in the TinyMCE toolbar
add_filter( 'wpforo_editor_settings', function( $settings ){
	$settings .= ',crayon_tinymce';
	return $settings;
} );

Still waiting on an explanation to this... the lack of response just makes it seem more deceitful 🙄 

So I managed to add tinymce plugins by adding them to the wpforo plugins folder, and editing the class-template.php file... that was really easy.  

Trying to do that via the functions file and I am struggling for some reason... have literally spent hours trying to work out why what you had don't was not working and I believe I have worked it out.

The filter does not work because the array is being reinitialised, effectively making the filter absolutely pointless.  

add_filter( 'mce_external_plugins', 'custom_mce_plugins');

function custom_mce_plugins( $plugin_array ) {
   $plugin_array = get_stylesheet_directory_uri() . '/tinymce-plugins/image/plugin.js';
   return $plugin_array;
}; 

The above is my fairly basic filter to load the plugin - basically the same as yours.

Array
(
     => https://staging.abcde.xyz/wp-content/plugins/wpforo/wpf-assets/js/tinymce-pre.js
     => https://staging.abcde.xyz/wp-content/plugins/wpforo/wpf-assets/js/tinymce-link.js
     => https://staging.abcde.xyz/wp-content/plugins/wpforo/wpf-assets/js/tinymce-spoiler.js
     => https://staging.abcde.xyz/wp-content/plugins/wpforo/wpf-assets/js/tinymce-code.js
     => https://staging.abcde.xyz/wp-content/plugins/wpforo/wpf-assets/js/tinymce-emoji.js
)

Array
(
     => https://staging.abcde.xyz/wp-content/themes/airi-child/tinymce-plugins/image/plugin.js
     => https://staging.abcde.xyz/wp-content/plugins/wpforo/wpf-assets/js/tinymce-pre.js
     => https://staging.abcde.xyz/wp-content/plugins/wpforo/wpf-assets/js/tinymce-link.js
     => https://staging.abcde.xyz/wp-content/plugins/wpforo/wpf-assets/js/tinymce-spoiler.js
     => https://staging.abcde.xyz/wp-content/plugins/wpforo/wpf-assets/js/tinymce-code.js
     => https://staging.abcde.xyz/wp-content/plugins/wpforo/wpf-assets/js/tinymce-emoji.js
)

add_filter( 'wpforo_editor_settings', function( $settings ){
	$settings .= ',image';
	$settings = 'true';
	return $settings;
} );

restriction is made by wpForo developers

@sofy Can you please advise how to actually add the plugins using the method you mentioned, because based on what @captain_planet said, it doesn't actually work. 

I didn't.  Unfortunately I ended up using a different forum plugin and writing my own Wordpress syntax highlighter plugin that uses prims.js.

Did you manage to work this out in the end? 

function register_crayon_tinymce( $plugin_array ) {
   $plugin_array = plugins_url() . '/crayon-syntax-highlighter/util/tag-editor/crayon_tinymce.js';
   return $plugin_array;
}
add_filter( 'mce_external_plugins', 'register_crayon_tinymce' );

function configure_tinymce_buttons($buttons) {
	return array('formatselect', 'bold', 'italic', 'subscript', 'crayon_tinymce');
}
add_filter("mce_buttons", "configure_tinymce_buttons", 0);

This restriction is made by wpForo developers so as to not allow the additional plugins, as it may cause the styling issue, etc.

If you're a developer you can simply add the TinyMCE additional plugins using the TinyMCE filters.