Basic outline here, to get started:
More complex implementation in the signature pad example, with an app to look at:
There’s a few other examples of using javascript in the forum, worth a search IMO.
Good that you’re aware of onload, note that the first time the javascript runs, the input element likely won’t be ready to be changed. The javascript in the HTML element will run at load, and again every time any dynamic values in it are updated.
I’ll be interested to see if autobinding does update the changed value to the database.
Happy cobbling!