TDocumentCustomProperty.FmtId Property
GUID for the property. In Excel this value is always {D5CDD505-2E9C-101B-9397-08002B2CF9AE} You can leave this null, in this case {D5CDD505-2E9C-101B-9397-08002B2CF9AE} will be used.
Syntax
Unit: FlexCel.Core
property TDocumentCustomProperty.FmtId: string