Open the Settings menu on the web or desktop apps, and enable the "Use Enter to Send Messages" option. If the said option is on, pressing the Enter or Return key on your keyboard will send the message. If you want to insert a new line while the option is enabled, press Shift + Enter or Ctrl + Enter.
If the "Use Enter to Send Messages" option is off, the Enter or Return key on your keyboard will add a line break/new line. To send the message while the option is off, use Shift + Enter or Ctrl + Enter.