add_action("cred_save_data", "func_set_product_external_by_default",10,2);
function func_set_product_external_by_default($post_id, $form_data) {
if ($form_data["id"]==9999) {
wp_set_object_terms( $post_id, 'external', 'product_type' );
}
}
Where:
Replace '9999' with your CRED form ID
You can present the form to your users to create a product and the using the Form's hook every time the product is created using that form, it will be set as a external product.