Actually, there is no Transfer Domain function in WP Domain Checker, but there is an alternative to achieve this. You can transform Whois Button into Transfer Domain button that linked to Woocommerce add to cart, WHMCS add to cart or external link.
In the WP Domain Checker settings:
On Whois Page option select Custom URL.
On Whois Custom URL put this URL:
WooCommerce add to cart:
http://yourwebsite.com/wp-admin/admin–ajax.php?add-to-cart=YOUR-PRODUCT-ID&domain={domain}
WHMCS add to cart:
*For Woocommerce add to cart, make sure you enable WDC in your Product data.
