Django REST Framework: default fields in browseable API form

I have a model:

class XCall(models.Model):
    created_on = models.DateTimeField(auto_now_add=True)
    send_on = models.DateTimeField(default=datetime.now)
    recipient = models.ForeignKey(User)
    text = models.CharField(max_length=4096)
    backup_calls = models.IntegerField(blank=True, null=True)

And a serializer for that model:

class CallSerializer(serializers.HyperlinkedModelSerializer):
    url = serializers.HyperlinkedIdentityField(
        view_name='call-detail',
    )
    # some validation and custom field definitions
    ...

    class Meta:
        model = XCall
        fields = ('url', 'id', 'text', 'recipient', 'send_on', 'backup_calls', 'status')
        lookup_field= 'pk'

And here's the list view:

class CallList(generics.ListCreateAPIView):
    serializer_class = CallSerializer
    permission_classes = (permissions.IsAuthenticatedOrReadOnly, IsOwnerOrSuperuser,)

    def pre_save(self, obj):
        auth_user = self.request.user
        obj.auth_user = auth_user

    def get_queryset(self):
        """
        This view should return a list of all the calls
        for the currently authenticated user.
        """
        auth = self.request.user
        if isinstance(auth, AnonymousUser):
            return []
        elif auth.is_superuser:
            return XCall.objects.all()
        else:
            return XCall.objects.filter(auth_user=auth)

In CallList's browseable API, I see the following in the POST form at the bottom:

My question is: why is there no default value set for send_on, and there is one for backup_calls? I assumed that the form would follow the XCall model specification and use datetime.now() for defaulting the former, and leave backup_calls blank (since it's nullable). How can I get the form to follow the model specifications?