Access to private field another class from specialized class

I have 2 classes, one is specialized. In the specialized class Class_second I would like to use private field server from Class_second. The constructor Class_first creates server connection and I would like to use this in another specialized classes.

class Class_first {
    friend class Class_second<Message>;

    Class_first() : server(create_conn(ip, port){

    }

...

private:
    unique_ptr<Server> server;

};

This is one of few specialized classes.

template<>
class Class_second<T> {
public:

    void foo(T) {
        Class_second.server.get();   //this is wrong, how to correct it
        ...
    }
};

How I can obtain access to the server field of Class_first object. What is the best way to use it?

Possibly how I can return the reference to the server and use it in Class_second?