https://cloud.google.com/pubsub/docs/create-push-subscription#pubsub_create_push_subscription-rest
PUT https://pubsub.googleapis.com/v1/projects/PROJECT_ID/subscriptions/SUBSCRIPTION_ID
Authorization: Bearer ACCESS_TOKEN
{
"topic": "projects/PROJECT_ID/topics/TOPIC_ID",
// Only needed if you are using push delivery
"pushConfig": {
"pushEndpoint": "PUSH_ENDPOINT"
}
}