How to use SQL Select statement with IF EXISTS sub query?

Use CASE:

SELECT 
  TABLE1.Id, 
  CASE WHEN EXISTS (SELECT Id FROM TABLE2 WHERE TABLE2.ID = TABLE1.ID)
       THEN 'TRUE' 
       ELSE 'FALSE'
  END AS NewFiled  
FROM TABLE1

If TABLE2.ID is Unique or a Primary Key, you could also use this:

SELECT 
  TABLE1.Id, 
  CASE WHEN TABLE2.ID IS NOT NULL
       THEN 'TRUE' 
       ELSE 'FALSE'
  END AS NewFiled 
FROM TABLE1
  LEFT JOIN Table2
    ON TABLE2.ID = TABLE1.ID