Chargebee - Reference Transcation - How to enable?

[hqapp]

HI,

I'm following setup instructions for chargebee paypal checkout.
https://www.chargebee.com/docs/paypal_express_checkout.html 

They mention I have to contact paypal support to enable Reference Transactions, but I don't understand how.
Can someone tell me how to go about this.

It's for this user-name: <removed>

Thank you,
Florian