Change or add parameters to the data string used by PayPal to get subscription details.

apply_filters( 'pmpro_get_recurring_payments_profile_details_nvpstr', string $nvpStr, MemberOrder $order);

Parameters

$nvpStr

string

$order

MemberOrder

Source

File: https://github.com/strangerstudios/paid-memberships-pro/blob/master/classes/gateways/class.pmprogateway_paypalexpress.php