Adding a Signature to Your Canvas Inbox

Click Inbox from the global navigation bar
Click the Inbox Settings icon
Check the Signature On checkbox
Enter what you’d like your signature to be. This will be applied to all newly composed messages in the Canvas Inbox. Note that at the time of this feature launching, rich-text editing is not available (i.e. no ability to format text or insert images)
Click Save
Now when you compose a message in the Canvas Inbox, your signature will automatically be applied.