Office 365 (Outlook on the Web) - Email Signature
Outlook Web Application users may create and set signatures to automatically append to the end of their outgoing emails or replies.
1. From the Outlook app, click the gear icon in the upper-right corner of the screen and select View all Outlook settings from the bottom of the dropdown menu.
2. Click Mail, then Compose and reply in the Settings menu on the left-hand side of the screen.
3. Edit and format your existing signature text in the email signature text box. If you would like to create a new signature, click the +New signature option above the signature text box.
4. Click the Save button.