Add Stripe Connect data to wc_stripe_payment_request

wc_stripe_generate_payment_request passes 3 arguments. Take a look at the source code:

/**
    * Filter the return value of the WC_Payment_Gateway_CC::generate_payment_request.
    *
    * @since 3.1.0
    * @param array $post_data
    * @param WC_Order $order
    * @param object $source
    */
    return apply_filters( 'wc_stripe_generate_payment_request', $post_data, $order, $prepared_source );

Try setting the correct amount of arguments to the filter hook, otherwise it won’t get called.

add_filter('wc_stripe_generate_payment_request', 'add_application_fee', 10, 3);