It's created dynamically by the server once you check-in with any custom check-in notes:
var checkinNoteFieldValues = new[]
{
new CheckinNoteFieldValue("Custom Note", "some value"),
new CheckinNoteFieldValue("Other Note", "other value")
};
var checkinNote = new CheckinNote(checkinNoteFieldValues);
var pendingChanges = workspace.GetPendingChanges(); // workspace is Microsoft.TeamFoundation.VersionControl.Client.Workspace
workspace.CheckIn(pendingChanges, "comment", checkinNote, null, null); // checkinNote will trigger a new CheckinNoteFieldDefinition