Add it as a custom config on the proxy host:

location / {
    proxy_pass http://192.168.1.123:4567;
    proxy_set_header Host $host;
    proxy_redirect http:// https://;
    proxy_http_version 1.1;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection $connection_upgrade;                
}

location = / {
    return 307 /s/blahblah/blahblah;
}