Hello world. Is this from Mailchimp?
Your Mailchimp Email UID is not set
      You can send Mailchimp Email UIDs in email shots by using the *|EMAIL_UID|* merge tag in a link URL. 
    
The URL used for this example was:
https://davidsimpson.me/labs/pii-viewer/mailchimp-example.html?mcid=*|EMAIL_UID|*
If your Mailchimp Email UID is set, then it will be sent over to Google Analytics (GA4) by this page using the following code:
const mcid = new URLSearchParams(location.search).get('mcid');
if (mcid) {
  gtag("event", "mailchimp_mailshot", {
    mcid: mcid,
  });
}