I've been searching and reading for a few hours and just can't find it.
In the Digikam settings of the metadata I checked all kinds of fields, and most of them I can them find back in the metadata editor (Shft-Ctrl-M) and whatever I fill in is shows in the right sidebar afterwards, but not like PersoninImage, Coverage, Publisher, Identifier. I can see them in the right sidebar, but I just can't find the place were to add them.