Support agents can make use of prewritten saved replies to speed up similar conversations. These are configured within an organization's settings - to edit them, see Manage saved replies for use in live support.
Click the bookmark button to the left of the text entry field at the bottom of the dashboard.
Alternately, you can open the menu by typing "#" into the text entry field.
Select your desired message from the menu. You can filter the menu by typing part of the message into the text entry field at the top of the menu.
This will not immediately send a message, but will instead populate the text entry field with the selected message. You can make any required edits before sending it to the user.