From their website (because I am a giver):
Southwest Airlines Pilots have parking privileges at the employee parking lot located on Elkridge Landing Road off the Airport Loop Road (MD Route 170). To park on this lot, please stop by the BWI Pilot Base and pick-up a hang tag and proximity card. Beginning September 1, 2015, the cost is $32 per month and will be payroll deducted. Please be sure to display your employee parking hang tag while you are parked on this lot. Please note, this parking area is patrolled by Maryland Transportation Police and they issue citations to any car not displaying a current hang tag. The hang tag expires every August 31 at midnight