This is a solution, you should to pass a default value in a field and use @extend_schema_field:

...
from drf_spectacular.utils import extend_schema_field

class MessageSerializer(serializers.ModelSerializer):
    ...

    @extend_schema_field(
        field=serializers.DictField(
            default={
        'message_id': 'eab1609b-e356-4b3b-ab4c-b1211ac8957c',
        'body': 'test',
        'file_name': None,
        'user_id': '23eac66e-ff97-423c-97c8-d34bff743355',
        'datetime_create': '2025-03-21T09:22:57.682112Z'
            }
        ),
    )
    def get_last_message(self, chat: Chats) -> Dict[str, Any]:
        ...