Hi,
You need to create an "item" custom field and not a product one.
For that kind of question, the HikaShop forum section will be more appropriated because your question is currently not related to HikaMarket.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.