using dropdown help in parameter

i m trying to get the dropdowm list into paramater in selection screen for bukrs ( ccode ) fetched from lips.

I m using following code.

PARAMETERS : z_p_burk TYPE bukrs OBLIGATORY

AS LISTBOX

VISIBLE LENGTH 4.

I want to see the ccode list when user clicks on the drop-down icon.

With the code mentioned above , I m only getting the icon but the list is empty !

What additional code should I write ??