It’s in your client area. It’s unique for every one and for every domain, so we cannot just give you some magic value to work with.
But this doesn’t solve the Cloudflare issue, because Cloudflare still have to be configured.
As for the is-a.dev domain, first of all, I don’t think it comes from GitHub. It was indeed on the public suffix list, but again you have to make sure that you can change its nameservers if you wanted to add it to Cloudflare.