In addition to customizing a publishable link, Trackable Links can be created under the Forms Publish page to track the source during registration.
To create trackable links, input Link Name for the Link URL:
After clicking the green "Create Link" button, the link will be created below with the 'utm_source'=[link name]:
To use the link, simply copy/paste the full link into your medium of choice. As registrations come in, the registration report will track the "Source" for each registrant. That report is available under Reports > New Report > Type = Registrations.