Question Can't set server directive in additional nginx config

[tdiroll]

I have a domain setup in plesk running a reverse proxy nginx server. In the "Additional nginx-directives" textfield I writing the following:

server {
server_name subdomain.mydomain.com;
location / {
proxy_pass http://mydomain:port;
}
}

But I get following Error:

nginx: [emerg] "server" directive is not allowed here in /var/www/vhosts/system/mydomain.com/conf/vhost_nginx.conf:1 nginx: configuration file /etc/nginx/nginx.conf test failed

I already tried to wrap it into a http{} scope but then it just says that "http" is not allowed. What am I doing wrong here?