Due to limitations of the Outlook application this can only be done using outlook.office.com 

1. On a web browser go to outlook.office.com → sign in as yourself

2. Click either your initials or picture set for Office (highlighted pink)

3. Click Open another mailbox - a new window will appear

4. Input the name/email address for the mailbox you want to add an automatic response to - Do not press enter this will close the box instead Click Open

5.  In the new tab created by opening another mailbox click the Settings cog (highlighted pink)

6. Click View all Outlook settings

7. Click Automatic replies

8. Use the toggle to Turn on automatic replies

9. Add text to the relevant boxes for either inside organisation or outside organisation

10. Click Save