nginx reverse proxy - proxy_pass leads to 503 service unavailable

i have to following configuration:

server {

    listen: 8080;
 
    location / {
        proxy_pass           https://somehost.abc.xyz;
        proxy_redirect       off;
        proxy_set_header     Host somehost.abc.xyz;
        proxy_set_header     X-Real-IP somehost.abc.xyz;
        proxy_set_header     X-Forwarded-For somehost.abc.xyz;
        proxy_set_header     X-Forwarded-Host somehost.abc.xyz;
        proxy_set_header     HELLO pizza;
    }

}

Whenever i hit the the proxy, let's say i go to http://localhost:8080/home , i get 503 service unavailable (not from the nginx service, from the app i'm trying to go to https://somehost.abc.xyz/home).

But if i just go to https://somehost.abc.xyz/home or do a redirect return 301 https://somehost.abc.xyz/home; its all good and i get to the app. Although this doesn't help me, beacuse i want to get there through the proxy so i can add a request header to every request that goes through.

Any ideas what could possibly cause this and how to solve it?

Try to use backend port on proxy_pass address, and is it header "HELLO" is valid to backend app?

server {
            listen: 8080;
     
            location / {
                proxy_pass          https://somehost.abc.xyz:443;
                proxy_redirect      off;
                proxy_set_header    Host                somehost.abc.xyz;
                proxy_set_header    X-Real-IP           $remote_addr;
                proxy_set_header    X-Forwarded-For $proxy_add_x_forwarded_for;
                proxy_set_header    X-Forwarded-Host    your.host.address;
                proxy_set_header    HELLO               pizza;
    
                
                proxy_ssl_server_name   on;
                proxy_ssl_name      somehost.abc.xyz;
                # Optional
                proxy_ssl_protocols TLSv1.1 TLSv1.2 TLSv1.3;
                proxy_ssl_ciphers   'Required SSL Ciphers';
                proxy_ssl_trusted_certificate   "/path/to/your/ca-cetificates.crt";
            }
        }

EDIT: using SSL Backend you should define proxy_ssl_name if you don't have backend ssl certs