Currently, Promotional URL does not work on a Multi-Site setup, as the domain used for the Promotional URL defaults to the Primary URL of the Primary Site. With this limitation, users would not be able to setup a Promotional URL for their other sites, as the domain of the Promotional URL does not change to the site's other domains even if the page it should go to is hosted on a different domain.
As an alternate solution, users can create a Web Site Redirect in place of the Promotional URL to be able to create a Promotional URL for their other sites.
Here is a sample scenario: "User has 2 web sites, http://www.mywebstore1.com and http://www.mywebstore2.com. The user can only create Promotional URLs for the Primary Web Store because of the limitiaton mentioned earlier. For the secondary site, the user can use a Web Site Redirect instead as their Promotional URL."
1. Navigate to Setup > Site Builder > Redirects > New.
2. Choose the domain of the secondary site (www.mywebstore2.com).
3. In the Redirect From URL field, enter a name that will complete the Web Site Redirect. This will serve as the counterpart of the Name in the Web Site Redirect. (e.g. testpromourl)
4. In the Redirect To URL field, enter the complete URL, including the additional parameters for the promotion (promo code, lead source, etc.) where the shopper should be redirected to when they access the Promotional URL. This serves as the counterpart of the page and Additional Parameters field in the Web Site Redirect. (e.g. http://www.mywebstore2.com/item1?promocode=100OFF)
5. Save the Redirect.
The user can now use http://www.mywebstore2.com/testpromourl as a Promotional URL for the secondary site, and provide this to their shoppers.