SSIS using variable in Lookup Transform

In the Lookup Transformation , I specify a reference data set to use as follow:

SELECT DISTINCT Client_ID 
FROM     dbo.CLIENT
WHERE  (ENROLLMENT_DATE >='2016-07-01') AND (DE_ENROLLMENT_DATE <='2017-06-30')

If I want to replace the hard-coded '2016-07-01' and '2017-06-30', does anyone know how I can do it? Thank you for your help.