To add a phrase to keywords, you need to write them in a hyphen. Wedding-ring. But interestingly, clicking on the link of this keywords combination on the model-sale-page, we find that the system does not index pages with this phrase, but indexes the presence of both words in the tegs!
This is nonsense, friends!
https://www.cgtrader.com/3d-print-models/wedding-ring?sort_by=newest