// enter your customized code here
MercatorUi.Forms.Sig.SigForm sigForm = (MercatorUi.Forms.Sig.SigForm)clickedButton.Form;
if (string.IsNullOrWhiteSpace(sigForm.CliRecord.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(sigForm.CliRecord.C_ID);