Есть клас для создания инстанса модели, в которой есть связь на пользователя. 
models.py
class Vendors(models.Model):
    COUNTRY_CHOICES = tuple(COUNTRIES)
    vendorid = models.AutoField(primary_key=True)
    vendor_name = models.CharField(max_length=45, unique=True)
    country = models.CharField(max_length=45, choices=COUNTRY_CHOICES)
    nda = models.DateField(blank=True, null=True)
    consent = models.DateField(blank=True, null=True)
    active = models.BooleanField(default=False)
    user_id = models.ForeignKey('c_users.CustomUser', on_delete=models.PROTECT)
    timestamp = models.DateTimeField(auto_now_add=True)
class VendorsCreateView(APIView):
    """Create new vendor instances from form"""
    def post(self, request, *args, **kwargs):
        vendor_serializer = VendorsSerializer(data=request.data)
        try:
            vendor_serializer.is_valid(raise_exception=True) 
            vendor_serializer.save(user_id=request.user)
        except ValidationError:
            return Response({"errors": (vendor_serializer.errors,
                                        )},
                            status=status.HTTP_400_BAD_REQUEST)
        else:
            return Response(request.data, status=status.HTTP_200_OK)
В postman в body передаю запрос в виде 
{
        "vendor_name": "Awaxddas",
        "country": "Belarus",
        "Primary Contact Name": "Jack Jhonson",
        "Primary Contact Email": "jack@gmail.com",
        "Secondary Contact Name": "Jack2 Jhonson",
        "Secondary Contact Email": "jack2@gmail.com",
        "Modules": "Module1, Module2",
        "NDA date": ""
    }
В Authorization использую Type: Basic Auth где ввожу логин и пароль. Но при сохранении получаю 
ValueError: Cannot assign "<django.contrib.auth.models.AnonymousUser object at 0x7f112a911f28>": "Vendors.user_id" must be a "CustomUser" instance.
Анонимный юзер