Property for Custom DAC field to hard code first character and now allow it to be overwritten?

Hello Community, 

I am working on a customization to add a custom user field with a character hard coded as the first character in the string and would like it to not be selectable\overwritable. So essentially it would look like “T” on the screen and start at data entry after that character. Currently I have below. Is there a property I could set to start the text entry at position 2? 

#region UsrTNo
[PXDBString(3)]
[PXDefault(typeof(T), PersistingCheck = PXPersistingCheck.Nothing)]
public virtual string UsrTNo { get; set; }
public abstract class usrTNo : PX.Data.BQL.BqlString.Field<usrTNo> { }
#endregion
public class T : PX.Data.BQL.BqlString.Constant<T>
{
      public T() : base("T") { }
}

Thanks!