Sending Arbitrary data with notification style Firebase cloud messaging

I have a short question. I want to send a custom value from a cloud function which listens for onWrite events in the Firebase Database. My Code for the Cloud function:

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);

exports.pushNotification = functions.database.ref('/chats/notifications/{pushId}').onWrite( event => {
  var values = event.data.val();

const payload = {
    notification: {
        title: values.username,
        body: values.message,
        sound: "default",
        displayName: values.username,
        UiD:  values.recieverUiD
    },
};
const options = {
    priority: "high"
};
return admin.messaging().sendToTopic(values.recieverUiD, payload, options);});

And now the database structure looks like this:

chats

-notifications

-some ID

-username: fdsdf

-uid: 9043554

How can I send the UiD to my device? the way like in the code does not work with displayName... how am I supposed to do it