QBO or QBD? It does not apply to QBO, only QBD. You do it by creating a new company under your accountant license in QBD, and then go through the process of adding payroll to that company (client) by entering your firm's credentials, which then associates the EIN with your payroll subscription. When in QBD and you select add payroll services, it will prompt you for an Intuit account to associate the payroll with, so that is why I say to enter your credentials. It will ask you to confirm and then voila, it is associated to your payroll subscription by way of your QBD accountant license.
I only use QBO Payroll for one client. Otherwise, it is simpler and cheaper to run payroll through QBD and simply do journal entries in QBO, as needed.