PO Selector for Custom Form

I have created a custom form which captures inventory id and supplier, based on those selections, I need to have a PO number selector which shows the relevant PO number. How can I modify this code snippet, because at the moment it shows all PO Numbers.

 #region PONumber
        [PXDBString(50, IsUnicode = true, InputMask = "")]
        [PXUIField(DisplayName = "PO Number")]
        [PO.RefNbr(typeof(Search2<POOrder.orderNbr,
            LeftJoinSingleTable<Vendor, On<POOrder.vendorID, Equal<Vendor.bAccountID>,
            And<Match<Vendor, Current<AccessInfo.userName>>>>>,
            Where<POOrder.orderType, Equal<Optional<POOrder.orderType>>,
            And<Vendor.bAccountID, IsNotNull>>,
            OrderBy<Desc<POOrder.orderNbr>>>), Filterable = true)]
        public virtual string PONumber { get; set; }
        public abstract class pONumber : PX.Data.BQL.BqlString.Field<pONumber> { }
 #endregion