Could not connect to Tag assistant from external host referrer in preview mode

Question

when I tried to open the preview mode link from email

I get this error

the only way I could connect is through tagassistant therefore the only initial referrers I could get are: "https://tagassistant.google.com/"

or empty referrer value, if I reload the page or copy-paste it into another tab

the reason I'm doing this is to test a trigger when the referrer value is a specific external website. How can I do that?