Track Conversions Through iFrames with Google Tag Manager
Create Google Analytics Settings variable with custom settings. Then, scroll down to Cross Domain Tracking section, set Auto Link Domains to { {Domains}} and Decorate Form to True like the screenshot below. Cross Domain Tracking Settings. Finally, head to Tags, add a new tag and name it as Universal Analytics Tag.
Google Tag Manager (GTM) Fishtank Consulting
The top one ( a.k.a. the