First, log into your company email account via outlook.office.com

Then, click on the gear icon near the top right of the window:

In the left side pane, select "Account" and then "Signatures"

Enter a name for your signature on the top line, what you want your signature to display on emails in the body section, then hit save at the bottom:

Lastly, select the signature you want to appear in new messages and replies/forwarded messages:

Note - if you do not want your signature to appear in replies/forwarded messages, select (No Signature)