You do not have to pay for parking or for tolls on ferries or bridges when you are a Louisiana National Guard member in uniform and under official orders, travelling to, engaged in or returning from official business. The exemption does not cover the Crescent City Connection bridges or ferries.
There is nothing to apply for. Be in uniform and carry your orders, and show them when you are asked to pay.
Your official orders and your military ID, carried with you while you are in uniform.