// geef hier uw customized code in
string c_id = clickedButton.Form.DataSource.Rows[0]["c_id"].ToString().TrimEnd();
if (c_id == "")
    return; // nieuwe, nog niet gevalideerde klant
MercatorUi.Forms.Billing.BillingForm billingForm = MercatorUi.Globals.Main.ShowBillingNewInThread(MercatorUi.Engine.Gescom.Billing.TypeVAEnum.V, 1, "Factu");
if (billingForm != null)
    billingForm.BillingEngine.ApplyCustomerSupplier(c_id);