Adding and using header (HTTP) in nginx

I am using two system (both are Nginx load balancer and one act as backup).

I want to add and use few HTTP custom headers.

Below are my codes for both;

upstream upstream0 {
    #list of upstream servers
    server backend:80;
    server backup_load_balancer:777 backup;
    #healthcheck
}

server {
    listen 80;
    #Add custom header about the port and protocol  (http or https)
    server_name _;

    location / {
        # is included since links are not allowed in the post
        proxy_pass "http://upstream0;"
    }
}

Backup system

server {
    listen 777;
    server_name _;
    #doing some other extra stuff
    #use port and protocol to direct
}

How can I achieve that?