I cannot seem to figure out how to use “custom cards” from 3. parties, with the new Lovelace editor.
A custom card need an “include” on the top of the page, but I cannot figure out how to do this when using the editor only version (earlier i did it via yaml mode and my own lovelace-ui.yaml script)
No, the raw editor is the complete Lovelace setup. Place the custom cards at the top of that text file. It is not the button “toggle editor” btw. It is at the right top of the page.