There isn’t a widely used, general-purpose EXIF/IPTC field specifically intended for “a URL about the subject of this photo.”

From the answers given, one XMP field that is explicitly URI-based is xmpDM:location, described as a location URL for a WebLink-type marker. However, that is fairly specialized and may not be recognized by most photo software for your use case.

If the URL is mainly for your own workflow, the practical answer is: pick a field and use it consistently, then read/write it with a tool such as ExifTool. Some people would use a general text field like ImageDescription, but that mixes the URL with descriptive content, which you said you want to avoid.

So the key consideration is software compatibility: if you need other applications to display or use the URL, first verify which metadata fields that software supports. Otherwise, a consistent custom choice in XMP is usually the safest approach.