To Set Up Microsoft 365 Auto Responder / Out Of Office Message using Outlook Web Access click here 

With Outlook, you can set up automatic replies for when you are out-of-office or unavailable to respond to email. 

1. Select File > Automatic Replies.
2. Select Send automatic replies.
3. Select Only send during this time range.
4. Choose the dates and times you'd like to set your automatic reply for.
5. Type in a message.
6. Select OK.