SQL: How do I enable optional parameters on a recursive table valued funtion?

I have a recursive table valued function in SQL Server 2005 that currently works great.  Now I want to make the parameter optional.  If @ID is null I want to assign it a value from a select statement.  I have tried to insert the following if statement into the body of the function many different ways and I get always get multiple errors:

IF (@ID IS NULL) SET @ID = (SELECT TOP 1 ItemCode FROM ConfigDropDownLists WHERE ParentID IS NULL AND isMapAggPnt = 'true')

What is the proper way to do this?

ALTER FUNCTION [dbo].[GetAggPointAndChildCoordinates]
(
	@ID VARCHAR(50)
)
RETURNS TABLE
AS
RETURN
(
	WITH Results AS
	(
	-- Anchor member definition
		SELECT cdl.ParentID AS ParentID, cdl.ItemCode AS ID, cdl.ListName AS [Type], cdl.ItemName AS [Name], cdl.Polygon, 1 AS [Level]
		FROM ConfigDropdownLists AS cdl
		WHERE cdl.ParentID = @ID AND cdl.IsMapAggPnt = 'True'
		UNION ALL
	-- Recursive member definition
		SELECT cdl.ParentID AS ParentID, cdl.ItemCode AS ID, cdl.ListName AS [Type], cdl.ItemName AS [Name], cdl.Polygon, [Level] + 1
		FROM ConfigDropdownLists AS cdl
		INNER JOIN Results AS r
			ON cdl.ParentID = r.ID
	)
 
	-- Statement that executes the CTE
	SELECT ParentID AS [ParentID], ItemCode AS ID, ListName AS [Type], ItemName AS [Name], 0 AS [Level],isnull(Polygon, dbo.GetAggregationPointPerimeterCSV(ItemCode)) AS Coordinates FROM ConfigDropdownLists WHERE ItemCode = @ID
	UNION ALL 
	SELECT ParentID, ID, [Type], [Name], [Level],isnull(Polygon, dbo.GetAggregationPointPerimeterCSV(ID)) AS Coordinates
	FROM Results
)

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