How do I post JSON data on a remote resource?
Quite a few ways to do that, but here's one approach using the queue api, for async processing:

Add the items to a queue on save.
Add a queue worker to process the queue and send the items to the remote service.

If you use a cron queue, you can process them automatically via cron.
More info:

https://api.drupal.org/api/drupal/core!core.api.php/group/queue/8.8.x
https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Annotation%21QueueWorker.php/class/QueueWorker/8.8.x