Multisite - Commerce reference between sites Setup

Main website: www.example.com
LMS website: lms.example.com


The Main website is public facing for visitors to browse the courses.
The LMS website is for the users/commerce to be managed and the actual Course material to be used/viewed.
Drupal Commerce manages the payments and Access modules manage the users access once purchased.


The 2 websites are setup as Drupal multi-sites:  


On the same server
Separate databases 
All modules/theme files are under the sites/all folder.


Question

How do I create a 'Buy' button on the main website that will take the visitor to the LMS website with that particular course added in their cart?

Note: there is only a hand full of courses so the link on the main website can be manually added for each course.
So I have discovered that I will need to create a Rule using the module Rules URL Argument that will take the URL /add-to-cart/[SKU] and add this to cart.

I can create a field on the main site for the SKU and this then outputs the link to the LMS website.

The source is here:
https://www.youtube.com/watch?v=6Rn7FLFytTU