PayPal Merchant Referral

Hello,
I'd like to add our PayPal Merchant Referral code into the payment buttons.
The button code for the membership plugin seems to be: https://www.paypal.com/cgi-bin/webscr
The PayPal Merchant Referral code is: https://www.paypal.com/us/mrb/pal=xxxxxxxxxxxx

PayPal states: Incorporate PayPal referrals into your payment tools
As a developer or solutions provider, you can integrate PayPal into your e-commerce application by inserting your personalized referral code into Website Payment buttons. This will help ensure that you're compensated for your referrals to PayPal.

How do we get your button to incorporate our merchant referral?
Thank you.