Customizing 'Quick ticket creation'

I’d like to add some additional fields to the 'Quick ticket creation’
portlet. I’d like to add some checkboxes along the bottom to set a
couple of custom fields that will then fire off some scrips when the
ticket is created. I see how to override the Elements/QuickCreate
template and have it display whatever I want, but how can I convert that
into something that will set the custom field?