If you want to disable first fast in the CustTransOpen query, you can do it with the QueryBuildDataSource object of the CustTransOpen datasource.
[FormDataSourceEventHandler(formDataSourceStr(CustTransOpen, CustTransOpen), FormDataSourceEventType::Initialized)]
public static void CustTransOpen_OnInitialized(FormDataSource sender, FormDataSourceEventArgs e)
{
QueryBuildDataSource custTransOpen_ds = sender.query().dataSourceTable(tableNum(CustTransOpen));
custTransOpen_ds.firstFast(false);
}