Change settings in Messages on Mac
Use Messages settings to set up and turn on iMessage, change options for sending and receiving messages, and turn on Shared with You.
Go to the Messages app
on your Mac.Choose Messages > Settings, then click an option at the top of the window:
General: Change general options such as the default text size or the sound that plays when you receive a message.
iMessage: Set up and turn on iMessage. Specify options such as how you can be reached for messages and if you want to send read receipts.
Shared with You: Turn on Shared with You for each app where you want to see content that’s been shared with you.
To learn about the options, click 
at the bottom of the window.