How do I access a field I created in a DAC Extension?

I am trying to add a new field to the Sales Order Entry screen.  I created a new field as a DAC Extension:

namespace PX.Objects.SO
{
  public class SOOrderExt : PXCacheExtension<PX.Objects.SO.SOOrder>
  {
    #region UsrCustomField
    [PXString]
    [PXUIField(DisplayName="Price Class ID", IsReadOnly = true)]
    public virtual string UsrPriceClassID { get; set; }
    public abstract class usrPriceClassID : PX.Data.BQL.BqlString.Field<usrPriceClassID> { }
    #endregion
  }
}

I have added the new field to appear on the screen of the Sales Order Form. I want to set the value of this field when the Location is changed.

I have the code to get the value I want to insert into it but I’m not sure how to access the field in the Event Handler under SOOrderEntry.

I tried: 

protected void SOOrder_CustomerLocationID_FieldUpdated(PXCache cache, PXFieldUpdatedEventArgs e, PXFieldUpdated InvokeBaseHandler)
    {
      if(InvokeBaseHandler != null)
        InvokeBaseHandler(cache, e);
      var row = (SOOrder)e.Row;

      if (row == null) return;

      // Find the Price Class for the Customer

      Location custLoc = PXSelect<Location, Where<Location.locationID, Equal<Required<Location.locationID>>>>.Select(Base, row.CustomerLocationID);

      // Set the value of usrPriceClassID to the PriceClassID from Default Location for Customer

      row.usrPriceClassID = custLoc.CPriceClassID;
    }

It fails to validate as usrPriceClassID doesn’t exist under SOOrder.  I think I need to access it via the extension but I’m not sure how.

Thanks for any advice.

Phil