If you have access to another mailbox, you can change the Out of Office / Auto Replies message(s) for it, turn replies on or off.
Go to https://outlook.office.com and log in as yourself (if not already logged in)
Click your icon in the top right corner of the window (will either be your picture if you've uploaded one, or your initials.)
Click Open another mailbox
Enter the name of the mailbox you want to change the message for
The second mailbox will open in a new tab/window. You can then update the message from there the same way as you would for your own mailbox -
Click the Settings cog (icon 4th from right)
In the Search box, start typing Automatic replies until it appears in the list, then click it.
Change the settings as required then click the Save button
You can then close the tabs if you no longer need them.