You want to use the extensibility app, Custom Fields and Logic, to define custom fields...

Custom fields can only be created for SAP Fiori apps that have been enabled by SAP for this type of extension.

You must release the SAP Fiori app for extensibility via transaction Setup Adaptation Transport Organizer for Key User Tools (S_ATO_SETUP).

You must release the SAP Fiori app for extensibility using the Enable Fields for Use transaction in the Custom Fields and Logic (SCFD_EUI) app.

There is no specific requirement. Custom fields can basically be created for any SAP Fiori app.