Modify the MemberOrder object before actually creating a subscription in the payment gateway. This passes the MemberOrder obejct and PMProGateway object.

apply_filters( 'pmpro_subscribe_order', MemberOrder $order, $this );

Parameters

$order

MemberOrder

$this

PMProGateway

Source

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