Office 365 (Outlook on the Web) - Create and Add a Signature
If you use both the Outlook Client and Outlook on the Web, you will need to create a signature file within each platform.
For the most current instructions, please check out the following Microsoft help article: https://support.office.com/en-us/article/Create-and-add-an-email-signature-in-Outlook-on-the-web-5ff9dcfd-d3f1-447b-b2e9-39f91b074ea3.