I don't use it behind Cloudflare, but I was able to get Nginx reverse-proxying traffic to Webflow. Here's part of that config:

  upstream webflow {
    server proxy-ssl.webflow.com:443;
  }

  location @webflow {
    resolver <some DNS server here>;
    proxy_pass https://webflow;

    proxy_set_header Host $webflow_host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Proto $corrected_proxy_scheme;

    proxy_ssl_verify off;
    proxy_ssl_session_reuse on;
    proxy_ssl_server_name on;
    proxy_ssl_name $webflow_host;
  }

Elsewhere (a Lua script), I set the $webflow_host variable and URI for the request, which is exec-ed from that Lua script (which is basically an internal redirect to the @webflow location block).