Setting up Cloudflare website with own nameservers?

Hey guys. Recently i saw a website using cloudflare but not using cloudflare nameservers. However, his nameservers pointed to a cloudflare IP Ex: ns1.website.com -> 173.245.58.50 ns2.website.com -> 173.245.59.4 (which are cloudflare ips) and the website has a cloudflare ip as well. Do anyone of you guys know how to do this? Thanks

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