Out of Office Via Web Mail (OWA – Outlook Web Access)
Go to the gear in the top right, left click on it.
Click “Set automatic replies”
Tick Send replies only during this time period, only if you want it to turn off automatically on a set date. Make sure Send replies to all external senders is selected. In bottom box (Send a reply once to each sender outside my organization with the following message) Type your message and click Save once done