You need to create an A-Record for the parent domain pointing to the required IP address.
Firstly, remove the sub-domain you added to the system and wait for the DNS information for that sub-domain to be removed from the system. If the system created A-Records for the sub-domain 'office' still exist you will be unable to create your own A-Record with the same name. You can confirm that these A-Records have been removed from the system by going to Domains -> Manage Domains, clicking on the small DNS link and checking the 'non-editable DNS records'.
Once you have confirmed that the system generated A-Records for the sub-domain are no longer there you can add your own A-Record in the same panel section.
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