Follow these directions:

  1. Click your display name in the upper-left corner of the screen
  2. Click All Settings
  3. Click Preferences
    Under the heading Composer turn on or off: "Use Ctrl + Enter to send a message"
    If you choose "Off", messages will be sent by pressing "Enter"If this option is turned off, use "Shift-Enter" to insert line breaks.

Watch a short video here.

Se also:

Sending, Message, Line break, send message