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.

url: /api/data/v8.0/accounts

json:

{
    "name" : "New account"
}

url: /api/data/v8.0/contacts

json:

{
    "firstname" : "New",
    "lastname" : "Contact",
    "[email protected]" : "/accounts(c6da77b6-d53e-e611-80b9-0050568a6c2d)"
}

As the parentcustomerid can be an account or contact you need to define the type of entity you want to set with _entityname after parentcustomerid.

url: /api/data/v8.0/quotedetails

json:

{
    "[email protected]": "/products(11c0dbad-91df-e311-b8e5-6c3be5a8b200)",
    "[email protected]" : "/quotes(69b5e1ae-037f-e611-80ed-fc15b428dcdc)",
    "[email protected]" : "/uoms(73a5daea-6ddc-e311-a678-6c3be5a8c0e8)",
    "quantity": 1
}