Tips can be included as fixed payments; throughout your account, per employee profile, or for specific events.
Account-wide
Go to Settings > Payroll > Custom Payroll Fields > choose 'Tip' and set the default value > hit 'Update to save the change.
Per employee
Head to the Employees module > select the employee > Pay rates > scroll down and set the default value for the specific employee > hit 'Save'.
Per event
Go to the Scheduling module > select an event > click the Payroll tab at the top (if you haven't added Tips to the event payroll table, click the pencil icon at the top right of the table and check the 'Tip' box and hit 'Update'). You will then be able to set event specific Tip values for each worker who has clocked in and out of the event.