Django, rest api post ForeignKey

I am writing REST API in Django, I create model Member where is ForeignKey from User table (auth_user)… method GET work perfectly but with post I have problem… every time I saw error message that fk_id cannot be null.

models.py

class Member(models.Model):
valid_from = models.DateTimeField(null=True, blank=True)
valid_to = models.DateTimeField(null=True, blank=True)
online = models.IntegerField()
vpn_ip = models.CharField(max_length=20, unique=True)
created = models.DateTimeField(auto_now_add=True)
fk = models.ForeignKey(User, on_delete=models.CASCADE)

@property
def username(self):
    return self.fk.username

@property
def password(self):
    return self.fk.password

@property
def email(self):
    return self.fk.email

@property
def status(self):
    return self.fk.is_active

@property
def admin(self):
    return self.fk.is_staff

@property
def fk_id(self):
    return self.fk.id

def __str__(self):
    return self.username

views.py

class MemberData(APIView):
serializer_class = MemberDataSerializer
permission_classes = [IsAuthenticated]

def get(self, request, format=None):
    '''
    Retrieves all Member from database
    '''
    data = Member.objects.all()
    serializer = MemberDataSerializer(data, many=True)
    return Response(serializer.data)

def post(self, request, *args, **kwargs):
    '''
    Put new member to database
    '''

    data = {
        "fk_id": request.data.get('fk_id'),
        "valid_from": request.data.get('valid_from'),
        "valid_to": request.data.get('valid_to'),
        "online": request.data.get('online'),
        "vpn_ip": request.data.get('vpn_ip'),
    }

    serializer = MemberDataSerializer(data=data)
    if serializer.is_valid():
        serializer.save()
        return Response(serializer.data, status=status.HTTP_201_CREATED)
    return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)

serializers.py

class MemberDataSerializer(serializers.ModelSerializer):
class Meta:
    model = Member
    fields = ["id", "fk_id", "username", "password", "email", "status", "admin", "valid_from", "valid_to", "online", "vpn_ip", "created"]

and If I try post data as application/json then I saw this error…

data sending in postman like this..

{
    "fk_id": 1,
    "valid_from": null,
    "valid_to": null,
    "online": 0,
    "vpn_ip": "10.40.10.3"
}

Exception Type: IntegrityError

Exception Value: (1048, "Column ‘fk_id’ cannot be null")