Sitefinity Ecommerce:Include Build Notation (BN) to Paypal S

Hi 

I am using sitefinity 8.0 version and i have built a paypal payflow payment method in my site for ecommerce trasactions.Now there is a requirement to include Build Notation codes into this paypal system.Here is some information on Build Notation codes given by paypal.

"BN codes (or Build Notation codes) are provided to Partners for inclusion into their product integration as a passed parameter during a transaction. BN codes provide tracking opportunities within the PayPal system on all transactions originating or associated with a particular Partner."

Below is the sample code that needs to be incorporated:

I have highlighted the line where BN code is included.

HTML Sample

<form action=www.paypal.com/.../webscr method=”post”>
               <input name=”cmd” value=”_cart ” type=”hidden“>
               <input name=”upload” value=”1” type=”hidden “>
               <input name=”business” value=”merchants@samplemerchant.com” type=” hidden“>
               <input name=”item_name_1” value=”Item Name 1” type=” hidden“>
               <input name=”amount_1” value=”1.00” type=” hidden“>
               <input name=”item_number_1” value=”12345” type=” hidden“>
               <input name=”shipping_1” value=”0.50” type=” hidden“>
               <input name=”paymentaction” value=”sale” type=” hidden“>
               <input name=”bn” value=”Your BN Code” type=” hidden“>
               <input value=”Checkout” type=” submit“>
</form>

Please let me know where should i go and include this change