Have a look at this:
If you want the rel-attribute on say, only external links, you could add that when the page is rendered. If the editor should be able to make a choice, that will of course not work.
My client wants to extend TinyMce "epi-link" and "EPiServer.Url" property with the ability to set "rel" attibute on both (read all links).
Their reference on why: https://www.ranktrail.se/blogg/google-infor-nya-nofollow-lankattribut/ ( in Swedish ).
But basicly add support for
rel="ugc" (user generated content)
This affects "Ranking credit" for links in Google Search for example.
Any idea how to do this?