The Source Default Field is Blank in Account Engagement Prospects

This means this field cannot always populate automatically since the information must come from the referrer URL in the visitor's browser. The browser must have cookies enabled, and the visitor must perform any tracking opt-in actions.

Common reasons for a blank source field value: 
   •   The visitor manually typed in your URL 
   •   The visitor visited your site from a browser bookmark
   •   Security software/extension/add on/proxy stripped out referrer info
   •   Visitor clicked from an HTTPS to an HTTP site
  •  A Account Engagement form iframed into the page loads before the tracking code
If the form iframe loads before the Account Engagement tracking code this can be seen under the ‘Prospect Activities’ with the form view showing before the page view. To prevent this from happening the Account Engagement Tracking code can be forced to load before the iframed form:
▸▸  https://help.salesforce.com/articleView?id=000318309&type=1&language=en_US&mode=1

A lot of popular sites are HTTPS now, so the lack of referrer information in that last scenario can be attributed to standard browser behavior when navigating away from an HTTPS site to an HTTP one. For example, if someone is on https://linkedin.com and clicks on a link over to http://example.com, the referrer information will be blank. As a result, there's no way for Account Engagement to know the source of the visit.

This is why it’s important to enable SSL to use a HTTPS Tracker Domain to reduce lost referring URL information. 

If you are using UTM parameters to populate Google Analytics's Source field, this will also populate the default Source field in the prospect. For more information, read our documentation that covers using the Google Analytics Connector.