Log in to your Outlook on the Web account and go to the Mail app

The main view of all apps in Office 365

Then, go to the gear icon at the top right-corner and click Mail

Go to the Option section in Outlook on the Web.

On the left pane, extend the Mail section, then Layout and select Email signature

Go to the Email signature section in Outlook on the Web.

Please note you can't copy and paste images into this area,

You need to manually upload them using the image icon in the text editor