How do I add tags that Calibre will see?

[abrogard]

GPT is telling me lies.  Commands it gives don't work.

I want to add a tag to a couple of hundred pdf files before importing to calibre.  a custom tag I suppose that is.  Just like when you add tags within calibre with whatever words you like.

But adding tags without a value given me by gpt results in no tag showing in calibre
Adding tag'with value' as given by gpt has exiftool reporting an error; the tag is not defined.

I've tried a few variations but I can't get it to work.  It can be done?

[StarGeek]

Exiftool can't write a custom tag without a definition telling it how to write it.

I do recall someone attempted to do so in this post but there was some problems. Part of it was that Calibre was using XMP qualifiers, which exiftool does not write.

Do you have a file that is filled out correctly that you can share? If not publically,if you can save it on Dropbox/GoogleDrive/etc and send me the link in a DM, I'll take a look and see what can be done.