This isn't a normal pop-up. The whole webpage changes to a new page and the message is in a small box in the middle of the page, with a black filter showing over the rest of the page. I've had it happen twice now and also ignored it. First time I closed the tab, second time I went back to a previous page.

You don't have to click on anything to get this particular page/ad. It just appears out of nowhere. I can't remember the details but the first time I saw it something made me think it was from Microsoft.