How to add extra key in created object in Django Rest Framework?

Question

I'm new to Django

Currently I'm trying to add a simple extra key on my returned json in the serializer. Here's the create() override of my serializer:

    def create(self, validated_data):
        available_dates = validated_data.pop("available_dates")

        # Default expiry time is 3 days
        expiry = datetime.datetime.now() + datetime.timedelta(days=3)
        event = Events.objects.create(**validated_data, expiry=expiry)
        for selectedTime in available_dates:
            AvailableDates.objects.create(
                event=event,
                **selectedTime,
            )

        signer = Signer()
        signedObject = signer.sign_object(
            {"expiry": expiry.isoformat(), "id": event.id}
        )

        return {**event, "signed_url": signedObject}

As you can see, I don't want to only return event, but I want to add a signed url.

This gave me an error: 'Events' object is not a mapping

So then how do I add extra key? What do I return in the create function? Can I create Response() as the return of create function in DRF?