Redirect "Add to Cart"

-- HikaShop version -- : 2.6.1
-- Joomla version -- : 2.59

The customer has a product which needs to be redirected to off-site registration and checkout. It is for a co-sponsored educational event. The event partner is handling reservations.
So the system shows me the actual path behind "Add to Cart."
But the path cannot be edited; so I am not sure why it is being shown to me. Since I could I see it, I assumed wrongly that I could simply edit it and save.
I want the path to be redirected from the product ID to here www.regonline.com/Register/Checkin.aspx?EventId=1802316
How can that be accomplished?
Please note:
I hid "add to cart" by setting quantity to zero. But now an annoying message shows: "No stock." Why is that message showing, and how to I get rid of it? This is a virtual product with no limit on quantity.
It seem that I can only hide the cart by setting quantity. But now this "no stock" message appears, causing confusion for the customer.
The reason I did the above is that I couldn't edit cart path. So instead I put the registration link in the product description.
Bits and pieces of this question are discussed tangentially in many prior posts. But no single post provides the clear answer I am looking for.
I am sure there is one, give the sophistication of Hikashop.
Thank you,
-Jim G.

Hi Jerome,
I gather it's not easy. May I know the trick to hiding the "No stock" message. Other posts indicated that I can hide the add to cart button by setting quantity to zero. But then the confusing "No stock" displays to customer. Simply hiding the cart button is probably the best solution in this case.
Thnx,
-Jim G.