I have asked a similar question in the past and Mrdenny was able to assist me with the information below:
That trigger would look something like this.
CREATE TRIGGER t_YourTable_u on dbo.YourTable
SET UserName = suser_sname(),
DateChanged = getdate()
WHERE inserted.PK_Column = YourTable.PK_Column
Table and column names will need to be changed to match your table and column names.
The table inserted is a virtual table who’s name can not be changed.
This worked great for me.