Log in to https://app.wipperoz.com/en
Hit "My wips > Customer dashboard" from the grey dropdown menu on the left.
Find the Wip related to the quote that received the message.
Hit the "View quotes" button, you will be redirected to the quotes related to the Wip
On the quotes page, find the quote and hit the "View private messages" button.
Reply the message or create a new one and hit the "Send" icon.
Note: These messages can only be seen by you and the freelancer who made the quote.