How to add "IF NOT EXISTS" to create trigger statement

I am new to sql server and procedures/triggers. I have the following code to create a trigger (it works):

CREATE TRIGGER [dbo].[Insert_WithdrawalCodes] 
   ON  [dbo].[PupilWithdrawalReason] 
   AFTER INSERT
AS 
BEGIN
    SET NOCOUNT ON;
        UPDATE [dbo].[PupilWithdrawalReason] SET DateCreated=dbo.SYSTEMTIME() 
        WHERE WithdrawalCodeID IN (SELECT WithdrawalCodeID FROM inserted)
END

How do I conditionally create only if the trigger does not yet exist? What am I doing wrong here? Stackoverflow has good examples of "if not exists", but I can't get this to work in conjunction with a CREATE. Here is one of my failed efforts:

IF NOT EXISTS (select * from sys.objects where type = 'TR' and name = 'Insert_WithdrawalCodes')
    CREATE TRIGGER [dbo].[Insert_WithdrawalCodes] 
       ON  [dbo].[PupilWithdrawalReason] 
       AFTER INSERT
    AS 
    BEGIN
        SET NOCOUNT ON;
            UPDATE [dbo].[PupilWithdrawalReason] SET DateCreated=dbo.SYSTEMTIME() 
            WHERE WithdrawalCodeID IN (SELECT WithdrawalCodeID FROM inserted)
    END
GO