POCreate_Extension.cs

//create protected access function that allows the graphs it is extending to access protected methods.
[PXProtectedAccess]
public abstract class POCreate_ProtectedExtension : PXGraphExtension<POCreate_Extension, POCreate>
{
    [PXProtectedAccess(typeof(POCreate))]
    protected abstract void LinkPOLineToSOLineSplit(POOrderEntry docgraph, SOLineSplit3 soline, POLine line);
}

//standard graph extension
public class POCreate_Extension : PXGraphExtension<POCreate>
{
    //protected method that is being overridden
    public delegate void LinkPOLineToSOLineSplitDelegate(POOrderEntry docgraph, SOLineSplit3 soline, POLine line);
    [PXOverride]
    public virtual void LinkPOLineToSOLineSplit(POOrderEntry docgraph, SOLineSplit3 soline, POLine line, LinkPOLineToSOLineSplitDelegate del)
    {
        del?.Invoke(docgraph, soline, line);

        //execute custom code
        POLineExt rowExt = line.GetExtension<POLineExt>();
        SOLine solineactual = SOLine.PK.Find(docgraph, soline.OrderType, soline.OrderNbr, soline.LineNbr);
        SOLineExt lineExt = solineactual.GetExtension<SOLineExt>();
        docgraph.Transactions.SetValueExt<POLineExt.usrHWSOPurchasingNote>(line, lineExt.UsrPuchasingNote);
        docgraph.Transactions.SetValueExt<POLineExt.usrHWSOPuchaseDeliveryOptions>(line, lineExt.UsrPuchaseDeliveryOptions);
        docgraph.Transactions.SetValueExt<POLineExt.usrHWSOPurchasePriority>(line, lineExt.UsrPurchasePriority);
    }
}