QuickBooks software is fulfill your all needs to grow your business. Manage your accounting with QuickBooks Payroll. When you buy or purchase a QuickBooks Payroll 
Accounting online, you will get a 16 digit unique payroll service key. This key is linked to Payroll and Employer Identification Number (EIN) service.
Find Payroll Service Key:Got service key, check your mailIn case, you have still now get any mail You can use the automated service key retrieval toolLast, Sign in through your Intuit and get activation key.
Learn how to enter/edit QuickBooks Payroll Service Key:If you not got any mail of QB payroll service key, Use automated service key retrieval tool. Sign in to Intuit account.
Open QuickBooks accounting, Select employeesChoose Payroll, enter payroll service key
Edit Service key:Open QuickBooks accounting, Select employeesSelect Payroll Service > Manage payroll service key
Final, Go to payroll service enter or edit keyPress next and click on finishReceive payroll update messagePayroll service is activeClick on OK to confirm the all changes.
Source: https://www.askforaccounting.com/how-to-get-quickbooks-payroll-service-key-perfect-solution-is-here/