Untitled

<iframe width="600" height="300" id="myFrame" src="https://play.gumlet.io/embed/649e698ab792a98fc2dc0791?watermark_text={username}"></iframe>

<p>Click the button to change the value of the Dynamic watermark</p>

<button onclick="myFunction('Gumlet')">Try it</button>

<script>
function myFunction(name) {
  document.getElementById("myFrame").src = "https://play.gumlet.io/embed/649e698ab792a98fc2dc0791?watermark_text="+ name;
}
</script>