To contact your clinic through the wawa app, you have to be connected to the clinic already. Read about how to get connected here.
Visit the "clinic" tab of the app
Click on "Messages"
To start a new message, click "Start new conversation"
Note that new conversations can only be started in the clinic chat opening hours, which you can see displayed on the Chat tab of the app
You can view closed/archived chats on the "Archived" page – you cannot reopen a closed chat, so you will have to start a new one.