Hi,
Is the order status "confirmed" ?
If yes, then I don't see why it wouldn't work. The link should appear automatically on the order details page on the frontend and the link will work. If the link doesn't work, it means that your server restrictions are too strong for the download process and likely it doesn't have enough time to send the file.
If no, then it means that HikaShop isn't receiving payment notifications from your payment gateway. Here is a documentation on that that you can follow:
www.hikashop.com/support/documentation/i...or.html#notification