To claim Employment Expenses CRA requires that your employer completes and signs a T2200 - Declarations of Conditions of Employment. This form states that you were required to use your vehicle for employment purposes. Also, if you receive an "allowance" (or reimbursement), the amount of the allowance must be included on your T4 as a taxable income.
If you receive an allowance that is considered to be "unreasonable", (IE: much lower than the CRA prescribed rates)and your employer "does not include it in your taxable income" you can you claim employment expenses, but must subtract the amount of allowance from your claim. You still need the T2200 form completed.
If you meet the requirements, you can them claim your the portion of lease payments, fuel, insurance, etc, that relates to your kms driven for employment.
Here's a link to the CRA's guide on Employment Expenses: https://bit.ly/2Tt9PWZ