In the "Get a custom URL" box, you'll see the custom URL(s) you've been approved for. You can't change the part in the gray box, but you may need to add a few letters or numbers to make the URL unique to you. More info here: https://goo.gl/YQKMnU . Hope this helps.
Got it. Definitely reach out to the Creator Support team who should be able to help with next steps: https://goo.gl/BxLi8H . Hope this gets resolved.