There is no such key in mojoPortal Web.config. It does not need to get that from Web.config. It always uses firstname.lastname@example.org and the password admin when setting up a new site except in related sites mode then the email will vary by site id but not the password.
There is no reason to make it come from web.config it keeps it simple for support that a new site always has the same username and password for the newly created user and it should be changed by the user to their own email and password right after the site is created. If it may vary by web.config it would be more difficult to provide support for people who don't know how to login after they create a new site or install mojoPortal.