The purpose is to fill manually every tag of every track of hundreds albums without having to right-click on each track to open the properties tab, which would be way too tedious.
See below for *showing* the Composer tag. But:
* You don't have to right-click each track to open properties. You can select multiple, and open Properties for all. Then selecting Composer will give you the opportunity to fill in.
* Do other data give you an indication on Composer? fb2k can populate Composer from other fields. But I do not bother to start explaining if you don't have those data ...
* You probably do not want to fill in manually if you can avoid it. I suggest the Musicbrainz Picard tagger. There is a "Classical extras" plugin, but that has some learning curve; try the default and see if it gets you what you want.
* Also, foo_discogs can retrieve tags from the Discogs.com website. https://www.foobar2000.org/components/view/foo_discogs .
I think you need to register a Discogs account to use it ... and you will get a Written-By somewhere, which is not the same as populating the Composer tag ...
To *show* Composer, you want a Composer column in playlists. When viewing the playlist - where you have column headlines like Album, Track no etc - right-click on one of them, and you will get a menu
Select Columns, and near the bottom, select More. Then you get a configuration page (which you could also have accessed through Preferences) with a "Custom columns" headline.
Add New. Give it pattern %composer%. Call it Composer (that is what _you_ will see). OK.
Then back to right-click a headline and select Columns>. Then you should see Composer on the list. Click it.
Then you should see Composer as one of the columns. You can move it to the left or right.