How to add glossary term to domain

Hello Gillerman!
Thanks for reaching out. 
My first guess is that none of the URLs you tried are valid because domains ≠ glossaries in the API model; domains belong to Unified Catalog, while glossary terms are managed via the Glossary (Atlas) APIs. 
I doubt that you can add a glossary term “to a domain” via a domain‑scoped URL. In Microsoft Purview, glossary terms are created independently using the Glossary (Data Map) REST API, and domain association is handled via metadata (not a /domains/{id}/terms endpoint). Could you try this
POST {purview-endpoint}/datamap/api/atlas/v2/glossary/term