Under Message options, locate the toggle for Send message using Enter/Return key.
If Send message using Enter/Return key is turned on, pressing Enter/Return on your keyboard will send your messages. To insert a new line, press Shift+Enter/Return.
If Send message using Enter/Return key is turned off, pressing Enter/Return on your keyboard will insert a new line. Press Shift+Enter/Return to send your messages.
Note: Send message using Enter/Return key is turned on by default.