When using OWA, you can choose to have your signature automatically populate every new email you compose. If you cannot see your signature when composing a new email, you might need to enable it on your signature settings.
To read more about this topic visit - https://www.wisestamp.com/guides/outlook-signature-not-showing/
1. Login to your OWA account and go to Settings > View all Outlook Settings > Compose & Reply.
2. Make sure to check the box ("Automatically include my signature on new message that I compose") to include your signature on new messages.