Platform Integration Development

The Editing H5P content section of the integration development page is blank.
https://h5p.org/creating-your-own-h5p-plugin

I can create an edit page along the lines of what the Wordpress plugin does.
Great that looks good - see screenshots.

On Wordpress I can see what it does server-side when the update button is pressed.
Calls PHP function : H5PContentAdmin::process_new_content
But how is the data getting submitted when the update button is pressed?
Presummably invoking Javascript in h5p-editor-php-library taken from github - but how / where?

What do I need to call when my save button is pressed?
There is no form in the H5P iframe and that is wrapped anyway by my own form (like Wordpress does).