I am trying to make this about:blank cloaking tab automatically add the ‘https://’ to the domain

My code is below, I’m kinda new to all this and I’m trying to make a bookmarklet and maybe it could auto add everything else before the domain? (www., ect)

(function() {
alert('a gamedevCCJ production');
    let url = prompt("about:blank cloaker please type your URL!");
    win = window.open();
    win.document.body.style.margin = '0';
    win.document.body.style.height = '100vh';
    var iframe = win.document.createElement('iframe');
    iframe.style.border = 'none';
    iframe.style.width = '100%';
    iframe.style.height = '100%';
    iframe.style.margin = '0';
    iframe.src = url;
    win.document.body.appendChild(iframe);
})();