The name you give your hub defaults to be the URL. I suppose you can type something different in the URL field as you're typing in your hub. However, once you get to the page where you add capsules, etc. The URL is set and you can not change it.
After you publish your hub your URL and a short version of it will show at the top of your page.
If you need the URL after that, you can always find it by going to your hub and looking in the address bar of your browser.