Add display method to table using Extension class

[Extensionof(tableStr(CustTrans))]
final class CustTrans_Extension
{
    //[SysClientCacheDataMethodAttribute(true)]  //This attribute will cache your display method.
    public static display CustPurchaseOrder SalesTableCustReq(CustTrans _this)
    {
        CustInvoiceJour invoicejour;
        SalesTable      salesTable;
        CustPurchaseOrder  PO ="";

        select invoicejour
            where invoicejour.InvoiceId == _this.Invoice
                && invoicejour.InvoiceAccount == _this.AccountNum
                &&  invoicejour.LedgerVoucher   ==  _this.Voucher
            &&  invoicejour.InvoiceDate ==  _this.transDate;
        if(invoicejour)
        {
            select * from salesTable
                where salesTable.SalesId == invoicejour.SalesId;
            PO=salesTable.PurchOrderFormNum;
        }
       return PO;
    }
}


Add a string control to form, change properties
Data method :CustTrans_Extension::SalesTableCustReq
DataSource:CustTrans
Cachemethod:Yes

4-Customize this method in your Form 

locate your form and create a new string to add this method in the form 

set the properties for the string as the below

Data method format must be “ExtensionClassName::MethodName”