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Change preferences in Messages on Mac
Use Messages preferences to set up and turn on iMessage, change options for sending and receiving messages, and turn on Shared with You.
In the Messages app on your Mac, choose Messages > Preferences, then click a preference pane:
General: Change general options such as the sound that plays when you receive a message and the default text size.
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 in a pane, click the Help button 
at the bottom of the pane.