I want to use $host variable in proxy_pass. If directly hardcode Hostname in proxy_pass it is working fine. But if I use variable $host it is giving me "502 Bad Gateway"

server {
listen 80;
server_name "";

location / {
proxy_pass https://cds.hh.net; //working fine
proxy_pass https://$host; //gives error. And $host value is same like above hostname "cds.hh.net"
proxy_redirect off;
}

}

Thanks for help.