ORACLE BEFORE INSERT EQUIVALENT IN SQL SERVER

I am having trouble creating an INSTEAD OF trigger in SQL Server to
replicate a BEFORE UPDATE trigger from ORACLE.
Here is a sample of the ORACLE BEFORE UPDATE trigger:
CREATE TRIGGER myTRIGGER ON MYTABLE
begin
:new.DT := SYSDATE;
if :new.NM is NULL then
:new.NM := USER;
end if;
end myTRIGGER;

It seems as though I have to jump through hoops in SQL Server AND I
cannot come up with correct results.

Here is a snippet from SQL SERVER (this is what I figured I needed to
do after reading various articles,questions):
CREATE TRIGGER myTRIGGER on THETABLE
INSTEAD OF UPDATE
AS

SELECT * INTO #MYTABLE FROM INSERTED

UPDATE #MYTABLE SET DT = GETDATE()
UPDATE #MYTABLE SET NM = USER WHERE NM IS NULL

UPDATE THETABLE
SET
DT = (SELECT DT FROM #MYTABLE),
NM = (SELECT NM FROM #MYTABLE)
WHERE THETABLE.ID = (SELECT ID FROM #MYTABLE)

Can anyone please shed some light on this? Thanks in advance.