Learn from my mistakes and save some time. I spent way more time on this than needed. If your referrer is missing from you Google Analytics and you use Google Tag Manager, then make sure you set the field alwaysSendReferrer to true per the screenshot below.
This field is necessary to always send the referrer as the dr parameter in web requests to Google Analytics. For more information on the referrer, see the following link.
Have a great day!