TinyMCE adds this if your link does not start with a / or http:// or https:// or similar.
You can override this value by setting document_base_url: https://www.tiny.cloud/docs/configure/url-handling/#document_base_url
You could also probably control this by adding your own callback method: https://www.tiny.cloud/docs/configure/url-handling/#urlconverter_callback
Or you could hook into the ContentSaving content-event and replace "/EPiServer/CMS/@" with "@" when the content is saved.
Why don't you just inject complete anchor tag from backend for some placeholder string like for example $$anchor?
I have a URL which I would like to add via @Model.Url in the TinyMCE rich text editor (not ideal I know). See image below. However Episerver CMS always auto prepends "/Episerver/CMS" to the link which turns the link invalid. Any way to avoid this behavior