Nowadays it is convenient to use the Asynchronous Tag at http://www.googleadservices.com/pagead/conversion_async.js that exposes the window.google_trackConversion function.

This function can be used at any time. For example after submitting a form, like in your case.

See https://developers.google.com/adwords-remarketing-tag/asynchronous/

Update 2018

Situation changed and it seems that you have more options now with the gtag.js: https://developers.google.com/adwords-remarketing-tag/