If you use Google AdWords you can track conversions by adding script into yout custom forms. To begin, click on the form icon at the top of your control panel and choose the option for form editor.

Click on your form name to access the settings for your custom form. Choose the gear icon to the top right of your form and choose the option for tracking.

Your script can be added to the bottom box under conversion tracking script.

To update your changes to your form, choose the button to save changes.