I've used this Sql Server trigger on the 'A' table of my centerlines for a number of years:
USE [vecc2012]
GO
/****** Object: Trigger [vecc].[centerlineseditor] Script Date: 8/3/2016 11:15:51 AM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
-- Author: <Author,,Name>
-- Create date: <Create Date,,>
-- Description: <Description,,>
-- =============================================
ALTER TRIGGER [vecc].[centerlineseditor]
ON [vecc].[a28]
for INSERT
AS update vecc.a28
--BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
--SET NOCOUNT ON;
-- Insert statements for trigger here
Set editor = user
where objectid in (select objectid from inserted)and user <> 'vecc'
--end
It just grabs the user-name to populate the field (unless the user is vecc; that's me); How does the the third party user name differ from what you are getting?
That should just about do it....