In order to use custom task identifiers or keys instead of the standard indexes and predecessor indexes within Gantt Chart Library for WPF or Project Management Library for WPF you need to define such custom properties at task item level and synchronize their values to the original properties used within databinding, and replace appropriate grid column bindings within the user interface view.
You can view a complete implementation example of the above within the Custom Identifiers sample application for the GanttChartTaskListView control.