LayoutProcessor.php

$jsLayout['components']['checkout']['children']['steps']['children']['shipping-step']['children']
['shippingAddress']['children']['shipping-address-fieldset']['children']['flat_number'] = [
    'component' => 'Snowdog_FourStepCheckout/js/view/form/element/abstract',
    'config' => [
        'customScope' => 'shippingAddress.custom_attributes',
        'template' => 'Snowdog_FourStepCheckout/form/field',
        'elementTmpl' => 'Snowdog_FourStepCheckout/form/element/input',
        'id' => 'flat-number',
    ],
    'dataScope' => 'shippingAddress.custom_attributes.flat_number',
    'label' => __('Flat No.'),
    'provider' => 'checkoutProvider',
    'visible' => true,
    'validation' => [
        'required-entry' => false,
    ],
    'sortOrder' => 79,
    'id' => 'flat-number'
];