TinyMCE adding data-attributes to links

I have to add data attributes to inline links (in the content) as well as custom classes - more accurately, I need to allow the client to do it without editing markup.

Problem one, adding the custom classes, I’ve tried adding the classes as documented, json, a json array - the docs say the classes are supposed to appear in a drop down - they don’t. I noticed the plugin was called ‘modxlink’ so I added the link plugin and link on a separate toolbar. still does not appear. I though maybe it was the skin - can’t change the skin from the installed modx one looking in the source I see default, dark, document and writer installed - the only one that works is modx.

Problem two, I can’t find any information on adding custom attributes to a link. Even if it is possible.

Does anyone know how to get these tasks done?