Navigate to mail.office365.com
Select the gear icon at the top right of the window.
At the bottom part of the list there is a Mail option under “Your app settings”
In the options list, under Layout, select Email signature
Enter your signature in the Box and tick the box “Automatically include my signature on new messages I compose”. Once ready, click save.
You can test to see if the signature works and how it looks by starting a new email