After writing your comment, click on the Reply button.
Your reply will be added alongwith the reply timestamp and you will be notified about it.
Please note that for some integrations, in app reply is not supported. This means you will be redirected to the review site, where you will have to sign in and then reply to a review.
Let us take an example of G2. There is a "Reply on G2" button. Click on it and you will be redirected to the G2 site where you will have to login with your G2 credentials and then reply to that review.
This feature is available with the following license :