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You can check my BLOG here

CREATE TABLE TestTable (id INT IDENTITY, string VARCHAR(100))
GO
CREATE TRIGGER TestTrigger ON TestTable
INSTEAD OF INSERT
AS
  IF OBJECT_ID('tempdb..#id') IS NOT NULL
    INSERT TestTable (string)
    OUTPUT inserted.id
    INTO #id
    SELECT CASE WHEN string = 'TestString' THEN string ELSE 'Stub' END
    FROM inserted
  ELSE
    INSERT TestTable (string)
    SELECT CASE WHEN string = 'TestString' THEN string ELSE 'Stub' END
    FROM inserted
GO

CREATE TABLE #id (id INT)

SELECT * FROM TestTable

INSERT TestTable (string)
SELECT x.string
FROM
  ( SELECT 'TestString' AS string
    UNION ALL
    SELECT 'AnotherString' AS string
  ) x

SELECT * FROM #id
GO