You need to have popup opened when you scrape with that command, also you can try to use different identifier when you scrape (it might be that page has more than 1 iframe, so <tagname="iframe"> could be picking one without SRC). I've tried to register for that site but I never received confirmation email. You can actually use the code bellow, once the popup is opened (so it's done in one node):
navigate($scrape attribute(<tagname="iframe">, "src"), "Wait")
This should open the iframe SRC in the browser instead of popup.