Try this format
Namespace/Module/etc/frontend/di.xml
<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
<type name="Magento\Checkout\Block\Checkout\LayoutProcessor">
<plugin name="custom-checkout" type="Namespace\Module\Plugin\PluginName" sortOrder="10"/>
</type>
</config>
Namespace\Module\Plugin\PluginName.php
<?php
namespace Module\Namespace\Plugin;
class PluginName
{
public function afterProcess(\Magento\Checkout\Block\Checkout\LayoutProcessor $processor, $jsLayout){
$test = 'test';
$newField = [
'component' => 'Magento_Ui/js/form/element/abstract',
'config' => [
'customScope' => 'shippingAddress.custom_attributes',
'customEntry' => null,
'template' => 'ui/form/field',
'elementTmpl' => 'ui/form/element/input',
'tooltip' => [
'description' => 'description'
]
],
'dataScope' => 'shippingAddress.custom_attributes.' . $test,
'label' => 'Custom Label',
'provider' => 'checkoutProvider',
'sortOrder' => 0,
'validation' => [
'required-entry' => true
],
'options' => [],
'filterBy' => null,
'customEntry' => null,
'visible' => true
];
$jsLayout['components']['checkout']['children']['steps']['children']['shipping-step']['children']['shippingAddress']['children']['shipping-address-fieldset']['children'][$test2] = $newField;
return $jsLayout;
}
}
To extend the private method Event-Observer should be used