Adding the 'Explicit' tag to an album in ITunes

Does anyone know how to add the explicit tag to an itunes album? I know how to add it to a song but not an album, like right beside the name of the album with the litter red E

I think this is an iTunes problem.
Mp3tag does not know anything about albums. And there is no entity to attach that tag to.

What to do with the advisory tag is described in the help:
I do not know what makes iTunes to rate an album as Explicit - how many tracks it needs for such a rating.
I am not even sure if this is not only available for albums that one bought in the iTunes shop.
See e.g. this thread on an iTunes page (external link):

So you have to keep investigating.