sendPrivateMsg.js

const userId = session.message.message.user.id;
const channelId = session.message.address.channelId;

// Load the specified user
firebase.database().ref(`chat-users/${channelId}/${userId}`)
    .once('value')
    .then((snapshot) => {
        // Get the ref data
        let user = snapshot.val();

        // Send a direct message
        bot.send(new builder.Message()
            .address(user.address) // Only users at this address will receive the message
            .text("Hi, this is a direct message"));
    })
    .catch((err) => console.log(err));