Canned responses, just like other messaging rules, need to be created on Guest experience.
To create a canned response for future usage:
Go to Guest experience.
Click on "Canned responses" on the sidebar.
Click "Add new" to create a new response.
You can also use short codes and custom codes in canned responses directly in your templates, just like any other rule. Those variables would be replaced by the actual values at the time you select them from the inbox.