You can display external website content as a borderless, padding-free popup by embedding an iframe in an HTML element.
Steps
Create a new display, and from the "Popup" type, select any template.
In the editor, delete all content in the template, and make sure the Form switch is turned OFF.
Click "Add Content" and add an "HTML" element.
Set the iFrame element's width and height attributes to your preferred values, add
style="display:block;", and paste it into the HTML content.
For YouTube, it would look like this:<iframe width="320" height="180" style="display:block;" src="https://www.youtube.com/embed/A85mx5KSHrA" title="Promolayer Debug Guide" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>
Go to Design settings > Size & position, then:
Enter the width value you set in step 4.
Set minimum height, margin, and padding all to 0.
Switch to mobile preview and set mobile width, height, and padding.
Click Save to finish.