WooCommerce - Add custom field to Checkout page

functions.php

<?php

/* ADD 'WEBSITE' REQUIRED FIELD TO CHECKOUT PAGE
----------------------------------------------------------------------------- */
//* Check if a specific product ID is in the cart
function hm_product_is_in_cart( $ids ) {
	// Products currently in the cart
	$cart_ids = array();
	// Find each product in the cart and add it to the $cart_ids array
	foreach( WC()->cart->get_cart() as $cart_item_key => $values ) {
		$cart_product = $values['data'];
		$cart_ids[]   = $cart_product->id;
	}
	// If one of the special products are in the cart, return true.
	if ( ! empty( array_intersect( $ids, $cart_ids ) ) ) {
		return true;
	} else {
		return false;
	}
}

//* Check if a specific product category is in the cart
function hm_category_is_in_cart( $categories ) {
	// Products currently in the cart
	$cart_ids = array();
	// Categories currently in the cart
	$cart_categories = array();
	// Find each product in the cart and add it to the $cart_ids array
	foreach( WC()->cart->get_cart() as $cart_item_key => $values ) {
		$cart_product = $values['data'];
		$cart_ids[]   = $cart_product->id;
	}
	// Connect the products in the cart w/ their categories
	foreach( $cart_ids as $id ) {
		$products_categories = get_the_terms( $id, 'product_cat' );
		// Loop through each product category and add it to our $cart_categories array
		foreach ( $products_categories as $products_category ) {
			$cart_categories[] = $products_category->slug;
		}
	}
	// If one of the special categories are in the cart, return true.
	if ( ! empty( array_intersect( $categories, $cart_categories ) ) ) {
		return true;
	} else {
		return false;
	}
}

//* Add the field to the Checkout screen
add_action( 'woocommerce_before_order_notes', 'add_website_url_field' );

function add_website_url_field( $checkout ) {

	$categories  = array( 'hosting-maintenance' );
	if ( hm_category_is_in_cart( $categories ) ) {

	    echo '<div id="website_url_field" class="woocommerce-additional-fields__field-wrapper"><h3>' . __('Website') . '</h3>';

	    woocommerce_form_field( 'hm_website_url', array(
	        'type'          => 'text',
	        'class'         => array('hm-website-url form-row-wide'),
	        'label'         => __('Website URL for Hosting & Maintenance'),
	        'placeholder'   => __('http://yoursite.com'),
	        ), $checkout->get_value( 'hm_website_url' ));

	    echo '</div>';

	}

}

// PHP: Remove "(optional)" from 'Website URL' description
add_filter( 'woocommerce_form_field' , 'remove_checkout_optional_fields_label', 10, 4 );
function remove_checkout_optional_fields_label( $field, $key, $args, $value ) {
    // Only on checkout page
    if( ! ( is_checkout() && ! is_wc_endpoint_url() ) )
        return $field;

    $optional = '&nbsp;<span class="optional">(' . esc_html__( 'optional', 'woocommerce' ) . ')</span>';
	$required = '&nbsp;<abbr class="required" title="required">*</abbr>';
    $keys = array( 'hm_website_url' );

    if( in_array($key, $keys) )
        return str_replace( $optional, $required, $field );
    else
        return $field;
}

//* Process the new Checkout field
add_action('woocommerce_checkout_process', 'custom_checkout_field_process');

function custom_checkout_field_process() {

	$categories  = array( 'hosting-maintenance' );
	if ( hm_category_is_in_cart( $categories ) ) {

	    // Check if set, if its not set add an error.
	    if ( ! $_POST['hm_website_url'] ) {
	        wc_add_notice( __( 'Please enter your Website URL for Hosting & Maintenance, even if the domain is not live yet.' ), 'error' );
		}

	}
}

//* Update the order meta with field value
add_action( 'woocommerce_checkout_update_order_meta', 'custom_website_url_field_update_order_meta' );

function custom_website_url_field_update_order_meta( $order_id ) {
    if ( ! empty( $_POST['hm_website_url'] ) ) {
        update_post_meta( $order_id, 'Website URL', sanitize_text_field( $_POST['hm_website_url'] ) );
    }
}

//* Display field value on the order edit page
add_action( 'woocommerce_admin_order_data_after_billing_address', 'custom_website_url_field_display_admin_order_meta', 10, 1 );

function custom_website_url_field_display_admin_order_meta($order){
	$websiteURL = get_post_meta( $order->id, 'Website URL', true );
	if ( $websiteURL ) {
	    echo '<p><strong>'.__('Website URL').':</strong> ' . $websiteURL . '</p>';
	}
}

?>