customization

To be able to use the constant string value in the PXSelector attribute you need to create the constant Bql class:

public class AttributeConstants
{
    public const string AttributeConstID = "MFGPRPALLE";

    public class attributeConstID : BqlType<IBqlString,string>.Constant<attributeConstID>
    {
        public attributeConstID() : base(AttributeConstID) { }
    }
}

After that, you can declare the field and put the PXSelector like the following:

public sealed class SOOrderExt : PXCacheExtension<SOOrder>
{
    public static bool IsActive() => true;

    #region UsrFakeField
    [PXDBString(10)]
    [PXUIField(DisplayName = "MFGPRPALLE")]
    [PXSelector(typeof(Search<CSAttributeDetail.valueID,Where<CSAttributeDetail.attributeID,Equal<AttributeConstants.attributeConstID>>>),new Type[]
        {
            typeof(CSAttributeDetail.valueID),
            typeof(CSAttributeDetail.description)
        })]
    public string UsrFakeField { get; set; }
    public abstract class usrFakeField : BqlType<IBqlString, string>.Field<usrFakeField> { }
    #endregion
}

Result: