If you cannot run code on the server, you will need to use an existing service provided by the server. (It is not a server unless it provides services).

One example of a service is a web server. If your server accepts http or https connections, you can send it data (packets) using that protocol.

You need to have an existing, usually permanent listening endpoint, in order to be able to send on-demand data.
This is what the server is supposed to provide.

When you have information on the port, protocol, and authentication required by that server endpoint, you can plan on how to connect to it.