I'm not quite understanding step 1 of the setup https://developers.cloudflare.com/cloudflare-for-pla

I'm not quite understanding step 1 of the setup https://developers.cloudflare.com/cloudflare-for-platforms/cloudflare-for-saas/start/getting-started/#step-1--create-fallback-origin .. what's the IP address if I'm wanting to point a custom subdomain at a worker?
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