To create a chat within the app you must have the Chat Admin permission enabled.
First step click the 3 dots (more... ) tab on the App. Then click manage chat.
Click "Create chat"
Select if you want to send by Sites, Departments, Sections or Users.
Example below is when using departments option, it will then provide you with a list of the departments within your site that you can select who to send the message to:
Type your message, or attach a file then slide the toggle on if you would like it to be Mandatory.
Click "Create"