Setting up CDN for 3rd party open source website

How can I utilize Cloudflare as a Content Delivery Network for a third-party open-source website in order to customize its static HTML, CSS and JavaScript files? The domain I’m trying to use Cloudflare as a proxy for is a third-party, and Cloudflare isn’t accepting the domain when I try to add it (the domain is for a virtual machine hosted by Coreweave, a cloud rendering service provider). They recommended that I use a CDN to serve the files but it did not give any more information about how to go about it.