Queries :: How To Populate RegionPrefix Parameter So It Returns All Of Data

In the following code I have a parameter 'RegionPrefix' that filters my data by the first 2 characters of the 'Queue' field. How do I populate the 'RegionPrefix' parameter so it returns all of my data? Currently if I leave it blank it returns nothing.

Code:
PARAMETERS RegionPrefix Text ( 255 );
TRANSFORM Format(Avg([Abandonment Rate]),"Percent") AS ABAN
SELECT [Call Summary Table].Year, [Call Summary Table].Month
FROM [Call Summary Table]
WHERE (((Mid([Queue],1,2))=[RegionPrefix]))
GROUP BY [Call Summary Table].Year, [Call Summary Table].Month
PIVOT [Call Summary Table].Queue;