pageHeader = document.getElementById("_otree-title");
pageHeader.parentNode.appendChild(document.createElement("hr"));
var el = document.createElement("div");
el.classList.add("my-2");
el.innerHTML =
  "<center><small><a href='mailto:abodens1@uni-koeln.de'>E-Mail</a></small></center>";
pageHeader.parentNode.appendChild(el);