The concept is a bit different than the other sites. You don't apply to the individual companies, but just sign up once for the platform. Then you see all shops that you are allowed to see right in iSecretShop. To accept a shop from a particular company, you must accept the ICA from that company, but at least so far, I have never been asked any additional application-like questions.
At the beginning, you can only self-assign one shop across the whole platform (not one per MSC). You receive points towards your "agent ranking" and that dictates how many shops you can assign as it increases. They don't offer much information on the site (that I could find at least) to describe the number of points needed for different rankings or what the associated privileges are.