[WooCommerce] How to add Custom Fields to Products

functions.php

add_action( 'woocommerce_product_options_general_product_data', 'woo_add_custom_general_fields' );
function woo_add_custom_general_fields() {

  global $woocommerce, $post;
  
  echo '<div class="options_group">';

	woocommerce_wp_checkbox( 
	array( 
		'id'            => '_no_free_shipping_checkbox', 
		'wrapper_class' => '', 
		'label'         => __('Exclude From Free Shipping', 'woocommerce' ), 
		'description'   => __( 'Dis-allow Free Shipping', 'woocommerce' ) 
		)
	);
	woocommerce_wp_checkbox( 
	array( 
		'id'            => '_discontinued_product_checkbox', 
		'wrapper_class' => '', 
		'label'         => __('Discontinued Product', 'woocommerce' ), 
		'description'   => __( 'No longer in Production', 'woocommerce' ) 
		)
	);
	woocommerce_wp_checkbox( 
	array( 
		'id'            => '_custom_product', 
		'wrapper_class' => '', 
		'label'         => __('Custom Order Product', 'woocommerce' ), 
		'description'   => __( 'Product can be customized', 'woocommerce' ) 
		)
	);	
	
	echo '</div>';
}

// Save Fields
add_action( 'woocommerce_process_product_meta', 'woo_add_custom_general_fields_save' );

function woo_add_custom_general_fields_save( $post_id ){
	
	// Checkbox
	$woocommerce_checkbox = isset( $_POST['_no_free_shipping_checkbox'] ) ? 'yes' : 'no';
	update_post_meta( $post_id, '_no_free_shipping_checkbox', $woocommerce_checkbox );
	
	$woocommerce_product_checkbox = isset( $_POST['_discontinued_product_checkbox'] ) ? 'yes' : 'no';
	update_post_meta( $post_id, '_discontinued_product_checkbox', $woocommerce_product_checkbox );
	
	$woocommerce_custom_product_checkbox = isset( $_POST['_custom_product'] ) ? 'yes' : 'no';
	update_post_meta( $post_id, '_custom_product', $woocommerce_custom_product_checkbox );	

}
add_filter('woocommerce_is_purchasable', 'ar_custom_is_purchasable', 10, 2);

function ar_custom_is_purchasable( $is_purchasable, $object ) {

    // get the product id first
	$product_id = $object->get_id();
	
	// get the product meta data
	$is_custom = get_post_meta($product_id, '_custom_product', true);
	
	if ($is_custom == "yes"){
		return false;
	}
	else {
		return true;
	}

}
add_action( 'woocommerce_single_product_summary', 'ar_custom_product_cta', 60);

function ar_custom_product_cta()
{
	global $product;
    // get the product id first
	$product_id = $product->get_id();
	
	// get the product meta data
	$is_custom = get_post_meta($product_id, '_custom_product', true);
	
	// Show the Form if product is Custom
	if ($is_custom == "yes"){
		echo '<h5> Questions? We respond in minutes! </h5>';
		echo do_shortcode( '[contact-form-7 id="19502" title="Custom Product Inquiry"]' );
	}
}