How to use SignalR to send data to a specific user?

I have a client which receives messages over SignalR. It is working great but it is more like a broadcast. I would like to be able to send messages to a specific client. On the client-side I have a userId and I set up my connection like this:

const userId = getUserId();

if (userId) {
    const beacon = new signalR.HubConnectionBuilder()
        .withUrl(`${URL}/api?userId=${userId}"`)
        .build();

    beacon.on('newMessage', notification => console.log);
    beacon.start().catch(console.error);
  }
};

On the server-side (Azure Function written in JavaScript) I have a message and a userId. The question for me is how does the server know which SignalR connection is going to this specific user? Can I somehow tell SignalR who I am?

Answer

Using the Azure SignalR Service and the client-side code from the question I was able to get it to work. I used the following Azure Function to negotiate the connection:

module.exports = async function (context, req, connectionInfo) {
  context.res.body = connectionInfo;
  context.done();
};

---

{
  "disabled": false,
  "bindings": [
    {
      "authLevel": "anonymous",
      "type": "httpTrigger",
      "direction": "in",
      "name": "req"
    },
    {
      "type": "http",
      "direction": "out",
      "name": "res"
    },
    {
      "type": "signalRConnectionInfo",
      "name": "connectionInfo",
      "userId": "{userId}",             // <----- IMPORTANT PART!
      "hubName": "chat",
      "direction": "in"
    }
  ]
}

As well as another function to send a message to a specific user:

module.exports = async function (context, req) {
  const messageObject = req.body;
  return {
    "target": "newMessage",
    "userId": messageObject.userId,
    "arguments": [ messageObject.message]
  };
};

---

{
  "disabled": false,
  "bindings": [
    {
      "authLevel": "anonymous",
      "type": "httpTrigger",
      "direction": "in",
      "name": "req",
      "methods": [
        "post"
      ]
    },
    {
      "type": "http",
      "direction": "out",
      "name": "res"
    },
    {
      "type": "signalR",
      "name": "$return",
      "hubName": "chat",
      "direction": "out"
    }
  ]
}