If you mean a field for a service, then it can be created by using the "Business/Corporate Settings => Groups of Services" menu. Each service belongs to a Group of services. For example, Translation is a group of services; English => German is the service, which belongs to the Translation Group. You can create your own groups, as well as services.

If you mean a Custom field, which is displayed, for example on the Custom tab of the Main Client window, then they can be created by using the "Advanced Settings => Fields" option.
Each custom field belongs to a group too. By default, custom fields are created within the Common group.