How do I claim the 400.00 deduction for working from home due to covid?

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Here are Canada Revenue Agency links with information regarding Expenses Tax Info Centre 

CRA  Eligibility criteria - Temporary flat rate method 

Note: For the flat rate method to be activated in TurboTax Online, you enter your T4 and scroll down to the bottom where you will be asked: Will you claim employment expenses for working at home due to COVID-19 for income on this T4? NEW!  You will then click on Employment Expenses found lower in the menu on the left side of the screen to then answer yes on the right screen. On the Employment Expenses Profile page, check the option: I didn't receive either form from my employer. On the next screen Check the flat rate method and enter the number of days you worked from home. You have now made your claim.

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