In the External Reference profile, you may specify Properties Database as the External Reference source type, then the value is set in the profile itself.
To set a value of an External Reference key, double-click on the value column for the specific row.

String Value Type

When Value Type is set to String the value dialog shows a single field to enter the value.

List Value Type

When Value Type is set to List the value dialog shows a table to enter values.

Map Value Type

When Value Type is set to Map the value dialog shows a table to enter Key and Value pairs.