Lightroom classic - query on IPTC fields

Many fields shown in LR's Metadata panel are not searchable, unfortunately.  But there are several plugins that will search those fields: Any Filter, Search Replace Transfer, and Data Explorer.

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link - this seems to offer some explanation

It may be possible, once you have found the actual field naming as shown in XMP, to copy that entry (before import / during metadata write, external update, re-read) into some other field which Lightroom will then expose to the user. For example: "Other Categories". Something like Exiftool could be configured with the right commandline options, that you can then drag-drop image files onto, in order to carry out that transfer.

Clearly going forward, directly pre-entering the needed info into some other field which LR will then support directly, is going to be a lot easier. Or entering the data within LR of course.