Open Outlook
Go to File
Select Automatic Replies
Choose Send automatic replies
Set the time range if needed
Type your auto-reply message
Add a reply for inside your organization if needed
Add a reply for outside your organization if needed
Click OK to save
Open Outlook on the web
Click Settings
Select View all Outlook settings
Go to Mail
Select Automatic replies
Turn on Automatic replies
Enter your message
Set the time range if needed
Click Save