You are required to log into distinctive Darknet markets with the only help of URL, that implies the following : upcoming vendees and vendors lack the ability to register until they will obtain an alternative links from an already signed in customer.
Those can be found at https://darkwebonion.com/, it possesses new URLs to all of the Dark Web marketplaces. You shall follow these alternative link to sign up at the sites.
In case that there are some troubles with an access (links don't work), you should hang around and check for Darknet markets alternative URL. Keep trying, as a several amount of the markets close and let signing up regularly, due to server load.