a way to store environment variable for custom package

Yes, you're right, on "after save", it will send data to my URL which has an API Key.

Thank you for your suggestion

I haven't seen it stated here but you can also access the config table in sidecar if the category is a module name.

var config = app.metadata.getModule('Accounts', 'config');
if(!_.isEmpty(config.ns_url)) {
    url = config.ns_url;
}

Note that you need to repair and rebuild for the client to get new values added to the config table.