It doesn't look like you do. There's a seat that fits there (on the real thing), and yeah, if you don't have it installed, the valve will leak. And while you will build boost, it'll be slow and low.
It looks like this: http://www.ebay.com/itm/Valve-Seat-T...-/350263172428
. Fits on top of the valve between it and the V-band. Lots of people actually install these V-band EWGs and forget to put the seat in, then post asking what they could have done wrong to make their EWG "open" at idle.