Outlook Web Access (Webmail)
If you have email hosted with Office 365 then you’ll have access to Outlook on the Web which also allows you to configure a signature when composing emails through the web interface.
To set this up log in to Outlook Web Access with your email and password. Once you’re viewing your Inbox, click on the Gear icon in the top-right then click View all Outlook settings. In the settings window click into Mail > Compose and reply then find the Email signature section, you can add or edit your signature in the text field there. You may want to click the checkboxes to automatically include your signature on new messages or on forwards/replies as well. Once you have it set up how you want it to appear, click the Save button in the bottom-right.
Now your signature should be available when sending email through the webmail with Outlook.
Pay special attention to the section Choose default signature on the right-hand side, if you have multiple e-mail accounts configured you will be able to select the account you wish to make signature changes to in that area. There are also two other options for New messages: and Replies/forwards:. Use these to set a default signature depending on whether you’re replying/forwarding or composing a new message, you can also set it to (none) if you’d prefer to manually add the signature on messages.
Outlook Mobile
Disclaimer: These steps were using the Outlook Mobile app for Android, however they should be very similar on the iOS app though they could differ somewhat
In Outlook mobile while viewing your mail, tap the three horizontal lines in the top-left of the screen. Then tap the gear icon in the bottom-left of the screen. Next look for the Signature option and tap it to configure the signatures. Set up the signature(s) to your liking and when done you can tap the check-mark icon located in the top-right of the screen.
Note: if you have multiple accounts configured in the app then you’ll have an option to configure signatures on a per-account basis. If you prefer to have a single signature for all configured accounts you can tap the switch icon in the top-right (just below the check-mark) to set it that way.
Pay special attention to the section Choose default signature along the bottom of the Signatures window, if you have multiple e-mail accounts configured you will be able to select the account you wish to make signature changes to in that area. There are also two other options for New messages: and Replies/forwards:. Use these to set a default signature depending on whether you’re replying/forwarding or composing a new message, you can also set it to (none) if you’d prefer to manually add the signature on messages.