exiftool

[mick crane <mick.crane@gmail.com>]

Hi,
exiftool is in the repository so I reckon is fair game for a question.
I'm sending a load of images to somebody and I want them to be 
Attribution-ShareAlike 4.0 International license
Don't want to plaster over images so think to add to exif data.
If type.
"exiftool -license="https://creativecommons.org/licenses/by-sa/4.0/"; 
./filename"
Gimp is complaining that metadata is not understood and do I want to 
import.
Gimp Metadata viewer is of the form "Exif.Image.Compression   
Uncompressed"
What is the correct form to add license to image with exiftool ?.
See talk of XML but I don't understand that.

mick