How to use nginx as a proxy

I'm having trouble setting up when using nginx as a proxy.
What should I do if I want the current 1 configuration to be like 2?

Include configuration file from nginx.conf

upstream proxy_server{
  server 54.248.71.136:3009;
}

server{
  listen80;

  location / {
    proxy_pass http://proxy_server;
    proxy_set_header x_custom_header_hoge "hogehoge";
  }
}

2. I want to do this
Client - > nginx - > Internet

In this case, how do I change the nginx setting?

nginx is a proxy for adding headers and will be explicitly specified on the client.
I would like to simply add a header and request it to the original request destination.
I feel like I have to specify a proxy destination in proxy_pass, so I wonder what I should do.
(I don't plan to use it as a reverse proxy)

Thank you for your cooperation.