class TestSerializer(serializers.ModelSerializer):
    profile = ProfileSerializer()

class Meta:
    model = Test

...
"relationships": {
                    "profile": {
                        "data": {
                                "first_name": "John",
                                "last_name": "Test"
                        }
                    }
                }
...

...
"relationships": {
                    "profile": {
                        "data": {
                            "type": "Profile",
                            "attributes": {
                                "first_name": "John",
                                "last_name": "Test"
                            }
                        }
                    }
                }
...