First of all: Welcome to the forums 
The thing is that UXP (XD’s platform for plugins) isn’t a browser and a lot of browser features aren’t fully supported. Among them are <iframe>'s and window locations.
Therefore, solutions depend on what you’re trying to do. If you want to include a fully functional “normal” web page, that’ll probably be difficult for now. If you need a screenshot of the website (like some other plugins), you could use an online service and then show the image. Simple information could be fetched using fetch() and then displayed. It really depends.
A feature request for embeddable web views (which you could vote for) is Embedded webview.
I hope this helps (although it’s probably not the answer you were hoping for),
Happy Coding,
Pablo