Boarding a new merchant account involves a few requests in succession to build up the account into a usable state. The requests are as follows.

Requests and its usage

/api/v4/merchantsTo create the merchant account.
/api/v4/processorsTo add a card/check processor.
/api/v4/processorsAgain to add value-added services.
/api/v4/merchantsTo set a fee schedule.
/api/v4/merchantsTo activate the merchant and send the welcome email.

There are also a few endpoints that are useful for pulling information that you will need for these requests.

/api/v4/usernames/searchLets you see if a merchant username is available.
/api/v4/services/searchLets you see what processors and value-added services are available to be added to merchants.
/api/v4/services/{{id}}/configLets you get configuration requirements for all processors.
/api/v4/fee_schedules/searchLets you get a list of fee schedules.