void BillingEngine_BeforeEinvoiceExport(object sender, MercatorUi.Engine.Gescom.BillingEngine.BeforeEinvoiceExportEventArgs e)
{
    MercatorUi.Engine.Gescom.BillingEngine billingEngine = (MercatorUi.Engine.Gescom.BillingEngine)sender;
    DataRow newLine = billingEngine.LIGNES.NewRow();
    Api.DataRowResetContent(newLine);
    newLine["id_article"] = "BIJDRAGE";
    newLine[billingEngine.VarQ] = 1;
    newLine["pu"] = 10;
    newLine["total"] = 10;
    newLine["taux_tva"] = 21;
    newLine["designatio"] = "Bijdrage";
    e.AllLines.Add(newLine);
}