Custom attributes in a Person or Group Column

Hi – I am in the process of migrating from SharePoint 2013 to SharePoint online, and I am having a problem solving this issue. Previously in SharePoint 2013 if I wanted a custom attribute to populate in the Person or Group column for a list (in this case employeeNumber), I would map this in ‘Edit User Profile Property’ in Central Admin and make sure it was replicable, which would then show in the  Person or Group column. I am trying to do the same thing in Office 365. I do not look after the Azure side of things, but have been told that it has been synchronised through  Directory Attribute Extensions Sync, but still cannot populate the Person or Group column. If anyone can help I would very grateful.