Discount Ninja allows you to set up both public and private promotions (read more about that here).
Shareable discount links are available for private promotions.
Here's how you create one:
Create a new promotion of type "Private discount"
Create a new promotion, select any type of "Private discount"
Fill in the details of the promotion using the wizard
Configure your pop-up, notification, sticky bar if required using the wizard
On the confirmation screen click "Edit promotion"
Build the shareable link
Find the "Shareable link builder" tab and use the wizard to create your link.
Read more about how to do that here.