Please note: that if your site uses a splash page, the coupon page link that you send your customer will not work. The reason it will not work is that all your pages are php based pages and your splash page is html based and not an actual integrated page within the software so it does not recognize the codes in the URL.

Click on Coupons under "E-Store" on the left hand side in your storeadmin.

Click on Add

 

Then fill out the form as shown in the example below.
Call the coupon whatever you would like and put in any coupon code #
you want, add an expiration date and fill in the % for the discount.
Click Update

 

Click on the Get Code button.

 

Copy the URL with the code at the end of it and send that to your customer.
When your customer shops from this URL, they receive whatever discount you specified.
ONLY those with this URL will be able to receive the discount.