How do I authenticate with ISP proxies?

Massive ISP proxies support three protocols:

curl --proxy http://isp.joinmassive.com:8000 -U '{PROXY_USERNAME}:{PROXY_PASSWORD}' https://cloudflare.com/cdn-cgi/trace

curl --proxy https://isp.joinmassive.com:4080 -U '{PROXY_USERNAME}:{PROXY_PASSWORD}' https://cloudflare.com/cdn-cgi/trace

curl -x socks5h://isp.joinmassive.com:8000 -U '{PROXY_USERNAME}:{PROXY_PASSWORD}' https://www.cloudflare.com/cdn-cgi/trace

Replace PROXY_USERNAME and PROXY_PASSWORD with your credentials.