Entity references are currently represented as a JSON object containing an @id member, e.g.

{
  "@context": "https://foo/bar/$metadata#$ref",
  "@id": "Orders(10643)"
}

An easier-to-create and also easier-to-digest representation would be to include instead all properties from either the primary key or one of the declared alternate keys):

{
  "@context": "https://foo/bar/$metadata#Orders(quux)/$entity",
  "quux": 10643
}

This is similar to the V4.01 format for deleted entities in delta payloads. If properties other than the key properties are supplied, these are ignored - a reference is not an implicit update.