Copy field from SO to Invoices

Hi every one,

I have a custom field on Sale orders named “So Notes” (usrSoNotes), I want copy it to Invoices (image).

I try code below, but still have data. 

public class ARRegisterExt : PXCacheExtension<PX.Objects.AR.ARRegister>
    {
        #region UsrSoNotes
        [PXDBString(800, IsUnicode = true)]
        [PXUIField(DisplayName = "So Notes")]
        [PXDefault(typeof(Search<SOOrderExt.usrSoNotes, Where<SOInvoice.refNbr, Equal<Current<ARRegister.refNbr>>,
             And<SOOrder.orderNbr, Equal<Current<SOInvoice.sOOrderNbr>>>>>), PersistingCheck = PXPersistingCheck.Nothing)]
        public virtual string UsrSoNotes { get; set; }
        public abstract class usrSoNotes : PX.Data.BQL.BqlString.Field<usrSoNotes> { }

  
        #endregion
    }

Thank for your help.