[SalesForce] How to properly send a Json in the body of a POST request

I'm trying to do the same I accomplish with postman but with Apex and I'm not being able to do so:

I need to send a POST request to this example endpoint:

https://cccccccccccccc/api_jsonrpc.php

This is what I've done so far:

First I've copied the json here: http://json2apex.herokuapp.com/

wich gave me this class:

public class JSON2Apex {

    public String jsonrpc;
    public String method;
    public Params params;
    public Integer id;
    public Object auth;

    public class Params {
        public String user;
        public String password;
    }

    
    public static JSON2Apex parse(String json) {
        return (JSON2Apex) System.JSON.deserialize(json, JSON2Apex.class);
    }
}

Then I've pasted that generated class as an inner class of my class (i've read it had to be done like this but I was getting strange errors so I built it apart, not as an inner class).

Then, I understand I need to use that generated class to parse my json and send it in the body of my post request but this is not working, I get this error:

Method does not exist or incorrect signature: void setBody(JSON2Apex) from the type System.HttpRequest

I undestand the error but I don't understand how to solve it. Any help or example would be more than valuable sience I've been googleling for a while now and can't understand it.

public class SendingZabbixServices {
    
    public void basicAuthCallout(){
        HttpRequest req = new HttpRequest();
        String endpoint = 'https://cccccccccccccc/api_jsonrpc.php';
        String method = 'POST';
        String jsonString = '{'+
            '   \"jsonrpc\": \"2.0\",'+
            '   \"method\": \"user.login\",'+
            '   \"params\": {'+
            '       \"user\": \"xxxxxx\",'+
            '       \"password\": \"xxxxxxxxxx\"'+
            '   },'+
            '   \"id\": 1,'+
            '   \"auth\": null'+
            '}';


        JSON2Apex obj = JSON2Apex.parse(jsonString);
       
        req.setBody(body);
        req.setEndpoint(endpoint);
        req.setMethod(method);

This is the json I'm trying to send in the body of the post request:

{
   "jsonrpc": "2.0",
   "method": "user.login",
   "params": {
       "user": "xxxxxx",
       "password": "xxxxxxx"
   },
   "id": 1,
   "auth": null
}