According to the google doc, a service running in the flexible enviroment can be the target of a push task:
Outside of the standard environment, you can't add tasks to push queues, but a service running in the flexible environment can be the target of a push task. You can specify this using the target parameter when adding a task to queue or by specifying the default target for the queue in queue.yaml.
However, when I tried to do it I get 404 errors in the flexible service.
That's totally normal due to the required endpoint (/_ah/queue/deferred) for task queues is it not defined in the flexible service.
How do I become a flexible service in a valid target for task queues?
Do I have to define that endpoint in my code in some way?