How to disable button new in markuptrans form if sales line invoiced ?

create extension from markuptrans form , and copy event handler for initialized form

  [FormEventHandler(formStr(MarkupTrans), FormEventType::Initialized)]

    public static void MarkupTrans_OnInitialized(xFormRun sender, FormEventArgs e)

    {

        FormDataSource  MarkupTrans_ds = sender.dataSource("MarkupTrans");

        SalesLine salesline;

        MarkupTrans  MarkupTrans;

        select SalesLine where SalesLine.RecId==MarkupTrans.TransRecId && SalesLine.TableId==MarkupTrans.TransTableId;

        if( SalesLine.SalesStatus == SalesStatus::Invoiced)

            MarkupTrans_ds.allowCreate(false);

    }