Custom HTML Tag.js

<script>
  (function(){
    var xmlhttp = new XMLHttpRequest();
    var db = "https://your-database.firebaseio.com/"+{{Client Id}}.replace(/\./g,"_")+".json";

    xmlhttp.onreadystatechange = function() {
      if (this.readyState == 4 && this.status == 200) {
        window.gaIds = JSON.parse(this.responseText);
      }
    };
    xmlhttp.open("GET", db, true);
    xmlhttp.send();

  })();
</script>