How to input (and where) amount from T2200 - section 8 : the amount of the allowance that was included on the employee's T4 sleep?

That info is already included on your T4 slip. 

 

If you have a monthly allowance then you would reduce your expenses claim by the amount of the allowance. Ex: If you get $300 a month for gas from your employer, you can only claim gas expenses that are over that $300 per month allowance - what you paid yourself.