Step 1: Select the account icon in the top right corner

Step 2: Select 'Account Settings'

Step 3: Select 'Message Signature'
Step 4: Enter your signature and/or default message and click 'Update.' This signature/message will automatically be appended at the end of all outgoing emails and can be edited in the Share panel prior to being sent.
Note: This signature shows up in the email message and pop-up box before the user opens the document.