All Mercedes Benz dealers have this list of CAN numbers and they can apply for you. You tell them you work for XYZ and show them a business card/paystub/badge and you fillout a form and you get teh discount.
You can also email the contact email at MB fleet which should be on their website to get the control number and the pin request form. From there you can simply email them proof that you work at your company and they will send you the completed form.
It can be applied to a demo and you should plan to negotiate your sales price with whatever car and dealer you choose before you mention you have fleet. Depending on the model and what incentives are available for November the pin may not add more value if the fleet discount is less or the same.