You can do this via exchange admin center.
1] use the exchange admin center to set up an automatic reply on user’s mailbox via using another user option.
1. sign in to your account in exchange admin center. go to the following url: https://outlook.office365.com/ecp/
2. in the navigation bar on the top of the screen, click on your name. a drop-down list will appear.
3. click another user.
4. select the mailbox that you want to open and click ok.
5. different window will open to allowing access on behalf of the other mailbox options and then you can set up automatic replies for the users.
2] there is another method but some administrative functions require administrators to access a user’s account via outlook web app.
this requires the user to have granted the administrator full access and send as permissions to his/her account. once an admin has full mailbox permissions you can then access a user’s account in outlook web app.
you can use the eac or the shell to assign permissions to users (called delegates) that allow to open other mailboxes. for more information: manage permissions for recipients.
after that you can able to set up an automatic reply on user’s mailbox via using open another mailbox.
1. sign in to your account in outlook web app.
2. in the navigation bar on the top of the outlook web app screen, click on your name. a drop-down list will appear.
3. click open another mailbox.
4. type the email address of the other mailbox that you want to open. another outlook web app session will open in a different window allowing access to the other mailbox and then you can set up automatic replies for the users.
for more information:
automatic replies (formerly out of office assistant)