Project StructuresYou want to enable user fields for customer project.

The benefit of assigning a field key to the project profile is that the field key is the default value for the WBS elements and the activities too. A field key is an object that defines which user fields are available and mandatory for a project object, such as a project definition, a WBS element, or an activity. A project profile is an object that contains various default values and control parameters for a project. By assigning a field key to a project profile, you can ensure that all WBS elements and activities created under that project profile have the same user fields settings. Reference:https://help.sap.com/docs/SAP_S4HANA_ON-PREMISE/4dd8cb7b1c484b4b93af84d00f60fdb8/df72bb53707db44ce10000000a174cb4.html