Upsert a Dynamics 365 Dataverse record using an alternate key instead of the primary GUID

Steps

1. Define an alternate key on the Dataverse table in the Power Platform maker portal or via solution import, specifying one or more column logical names that together form a unique key
2. Wait for the alternate key status to show as Active — key creation indexes the table asynchronously and can take minutes to hours for large tables
3. Construct the upsert request URL using the alternate key syntax: /api/data/vX.X/<entitysetname>(<keyname>=<value>) for a single-attribute key, or (<key1>=<val1>,<key2>=<val2>) for a composite key
4. Send a PATCH request with the record data as a JSON body; include the If-Match: * header to update only if the record exists, or omit the header to allow creation if not found (true upsert behavior)
5. On a 201 Created response the record was inserted; on 204 No Content the record was updated — use these status codes to track create vs. update for downstream logic
6. Handle 412 Precondition Failed when If-Match: * is used but no matching record exists, and 404 when the alternate key value does not match any record