You might be seeing an error saying "This website refused to connect" error message when trying to pop up a webpage. This error on the Webpage Popper means that the webpage you're trying to share does not allow itself to be shared this way.
If you own the website, then you or your webmaster can change the iframe settings so Meetn can show it. However for webpages you don't own, there's nothing you can do other than ask them to change that setting.
The Webpage Popper is designed for sharing your own webpages specifically, since you can fully control their usage. Generally speaking though, we recommend trying out sites you want to share on https://iframetester.com/. If it works on the tester, it will work in Meetn too.