What are the web services that require the obligatory creation of a subdomain?

Question

Computer networks

I am designing an automated creation of a web application. It is necessary to understand the logic and optimal steps with the least number of barriers for the user.
Tell me, what are the examples of services where, in order to use them, you must first go through the creation of your account with the subdomain name on this service?

Answer 1

LJ, slack?