onExtensionAfterSave isn't a common Joomla event - It only fires when the component/module/plugin configuration is saved. Example, editing Joomla global configuration, editing the options of a component, editing the options of a module, editing the options of a plugin. I've been writing extensions for years and I've never used that event - because it just isn't useful.
I think you picked the wrong event.
It won't fire on creating a form within your component unless you've written your component to fire that event. And if you did, I'd suggest picking a different event - like onContentAfterSave, which is designed to do what you're asking for.
If it's your own component (that you're writing) - you might consider creating your own set of events to monitor. Joomla is designed to do this. I did this in one of my own components to monitor a dozen different events related to CBT training.
When you're working with your own events, you can define whatever you want to send to the plugin. In my events, most take only one variable - an ID. The plugin retrieves the data based on that ID and does whatever it's designed to do.
Step 1, choose a unique type to name your plugin folder: /plugins/YOURNEWTYPE/
Step 2, make a test plugin to define and test your events.
Step 3, load the plugins and create a dispatcher in your component to fire your custom event whenever you need it.
It's super easy.