Please do as follows to allow proration on your bonus:
Go to the Payroll Module, Pay Record, and click New.
Select Record type as Bonus Pay.
Click the Prorate Factor tab and select the type of calendar proration.
Enable the Prorating setting, using Hire Date and Cessation Date, if you want the system to prorate the employee's incomplete year.
Click Save and click Next.
Move the employee over and click Confirm.