How put a variable inside Quotes in javascript that will be a html element

Question

So page is a string and id like to put video inside it but im not sure how to put the ytVideo variable as the src, the code I have right now doesn't work, anyone know how to fix it?

    var ytVideo = prompt("Please enter a link", "https://www.youtube.com/watch?v=CAyWN9ba9J8");
    page += "<iframe width=420 height=315 src=ytVideo></iframe>";