Where to put the logic for filling out fields of an object during post request?

Question

I am currently working on a post request for an image labelling game. Basically a user should see a picture that is shown to them through a get request and the user can label this picture (the resource).

This id my serializer:

class TaggingSerializer(serializers.ModelSerializer):
  tag = TagSerializer(required=False, write_only=False)
  resource_id = serializers.PrimaryKeyRelatedField(queryset=Resource.objects.all(),
                                                   required=True,
                                                   source='resource',
                                                   write_only=False)
  gameround_id = serializers.PrimaryKeyRelatedField(queryset=Gameround.objects.all(),
                                                    required=False,
                                                    source='gameround',
                                                    write_only=False)
  user_id = serializers.PrimaryKeyRelatedField(queryset=CustomUser.objects.all(),
                                               required=False,
                                               source='user',
                                               write_only=False)

  class Meta:
    model = Tagging
    fields = ('id', 'user_id', 'gameround_id', 'resource_id', 'tag', 'created', 'score', 'origin')
    depth = 1

  def create(self, validated_data):
    """Create and return a new tagging"""

    tag_data = validated_data.pop('tag', None)
    if tag_data:
      tag = Tag.objects.get_or_create(**tag_data)[0]
      validated_data['tag'] = tag

    tagging = Tagging(
      user=validated_data.get("user"),
      gameround=validated_data.get("gameround"),
      resource=validated_data.get("resource"),
      tag=validated_data.get("tag"),
      created=datetime.now(),
      score=validated_data.get("score"),
      origin=validated_data.get("origin")
    )
    tagging.save()
    return tagging

  def to_representation(self, instance):
    rep = super().to_representation(instance)
    rep['tag'] = TagSerializer(instance.tag).data
    return rep

and this is my post request:

    def post(self, request, *args, **kwargs):

        tag_serializer = TagSerializer(data=request.data)
        tagging_serializer = TaggingSerializer(data=request.data)

        if tagging_serializer.is_valid(raise_exception=True):
            tagging_serializer.save(tagging=request.data)
            return Response({"status": "success", "data": tagging_serializer.data}, status=status.HTTP_201_CREATED)
        else:
            return Response({"status": "error", "data": tag_serializer.errors}, status=status.HTTP_400_BAD_REQUEST)

At the moment, testing my post request works with such an object:

{
        "user_id": 1,
        "gameround_id": 2015594866,
        "resource_id": 2975,
        "tag": {
            "name": "NewTagToTestTheoryThatblabla",
            "language": "en"
        },
        "score": 0,
        "origin": ""
    }

I think for my use case I need to be able to send such a json object:

{
   "name": "NewTagToTestTheoryThatblabla",
    "language": "en"
}

where the other fields are filled out with data that was previously sent through the get request. Correct me if I am wrong here.

My question is, how can I achieve this?

*I have tried writing the logic in the post method and it doesn't work so I suspect I have to write it in the Serializer - since this is what I am doing for the 'created' field for example.