invoiceninja/app/Ninja/PaymentDrivers/PayPalProPaymentDriver.php

<?php namespace App\Ninja\PaymentDrivers;

class PayPalProPaymentDriver extends BasePaymentDriver
{
    public function gatewayTypes()
    {
        return [
            GATEWAY_TYPE_CREDIT_CARD
        ];
    }

    protected function paymentDetails($paymentMethod = false)
    {
        $data = parent::paymentDetails();

        $data['ButtonSource'] = 'InvoiceNinja_SP';

        return $data;
    }
}