wcus_ttn_form_payment_method

/**
 * @var string $paymentMethod - Current shipment payment method (default: Cash)
 * @var \WC_Order $order - WooCommerce order for which TTN is creating
 */
add_filter('wcus_ttn_form_payment_method', function ($paymentMethod, $order) {
    return 'Cash';
}, 10, 2);