I can add a keyword and create the XMP: Subject:
./exiftool -P -overwrite_original -xmp:subject='first tag' "/volume1/uploads/Phone_Jameson/"
I have tried variations of:
./exiftool -P -overwrite_original -xmp:subject+=${CreateDate} "/volume1/uploads/Phone_Jameson/"
I also dont know what the $ and {} are used for. just saw them in my searches for a solution.
I would also like to limit it to just the year and be able to combine it with some text so the keyword would display as "Year: 2019".
Figured it out finally.
"-xmp:subject<CreateDate"
or for just the year,
-d %Y "-xmp:subject<CreateDate"
Needed the " around everything and < symbol
And with a little more trial and error I got the Year: 2019 I was looking to achieve with ' around everything:
-d %Y '-xmp:subject+<Year: ${CreateDate}'
Yep, this is Common Mistake #5c (https://exiftool.org/mistakes.html#M5).
And don't forget to use +< if you want to avoid overwriting existing keywords.