At the moment, you have to pass the company key in the registration URL to let people do that. This is a really bad user experience & I expect this to be changed really soon. Probably next week.
This has to be done because people may have multiple companies & we should know which one registration page is pointing to. However, the new implementation will show the “Register” button on the login page that will lead to the default (and only if you have one) company.
Also, at the point where we introduce this to the official hosted platform, you should be able to manage multiple companies’ registration with subdomains.
TLDR: This is the format domain.com/client/register/COMPANY_KEY