You can find this section in Settings » Products & Services » Order page » Product » Components »Domains
Require hostname for this product - with this option enabled your customers will be required to enter hostname during this product signup.
You can bulk update your products to require hostname in given category by running this query (replace CATEGORY_ID with actual category ID taken from admin category listing page URL):
UPDATE hb_products SET domain_options=1, hostname=1 WHERE category_id = CATEGORY_ID |