Executing js before AutoPostBack of checkbox

NET 4 and asp checkbox which does an auto post back inside a user control and I need to call a js before that autopostback occurs.

Editing the onclick attribute of InputAttributes was of no help. It doesn't let me do that.

Tried registering on submit event or startupscript which was of no help. Unless there's a way to register on submit when checkbox was clicked or unclicked. Haven't found this yet. When I do register on submit my function gets called whenever anything happens on the page.

Don't want to start including jquery on my page just for this.

My last option would be to submit the form in js without AutoPostBack, however would love a solution using Register on submit or register startup instead of this. I want to be consistent with my code.

Thanks -P