Get Current Logged-In Customer Details

I am trying to add embedded content to a site, but I need to make an AJAX request to get the embed URL for the iframe. This request needs to provide a unique identifier for the current logged in customer. Basically like this:
 

(function() {

  var fd = new FormData();
  fd.append('userId', /* somehow get a stable customer id? */);

  var xhr = new XMLHttpRequest();
  xhr.open('POST', myApiUrl, true);
  xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
  xhr.onreadystatechange = function() {
     // ...status checks...
     var { uri } = JSON.parse(xhr.responseText);
     var frame = document.createElement('iframe');
     frame.src = uri;
     document.getElementById('myTargetElement').appendChild(frame);
  }
  xhr.send(fd);

})();

Any suggestions? I noticed that window.Static.SQUARESPACE_CONTEXT.authenticatedAccount provides all of the info that I need, but doesn't appear to be populated for member accounts.

Hi,
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