How to enter mileage expense with a professional vehicle
Entering a mileage expense with a professional vehicule is exactly the same as entering it with a personal vehicle, you just need to make sure that your admin has entered the vehicle as professional on Expensya, and that it's assigned to you.
You should not expect reimbursement since you are using a company vehicle
On the mileage expense form, a 'Pro' badge next to the vehicle name will let you know.

The main function will be to allow your employer to keep track of company vehicles' traveled distances, so no reimbursement are to be expected.