url: /api/data/v8.0/annotations
json:
{
"isdocument": true,
"mimetype": "text/plain",
"documentbody": "dGVzdA==",
"[email protected]" : "/accounts(c6da77b6-d53e-e611-80b9-0050568a6c2d)",
"filename": "test.txt"
}
As the objectid can be almost every entity in CRM you need to define the entity with _entity name after objectid.