Hey @Sadie1! The
fantastic-author-280 prefix is your account domain, which can be changed from your settings page2. You can also customize the
51cb0f103e part per page from its builder page (Settings > Domain Name > Page URL).
Log in to ask questions about domains publicly or anonymously.
I had this problem too. There's a little plus sign to the left of Google's ADD button. Click that and you can put in the other two values.