The $400 Employment expense related to a home workspace can be found on line 22900 under Other Employment Expenses.
However, before entering those amounts, you have to go to your T4 Employment Income on TurboTax, scroll down to the bottom of the page, where you will be asked "Will you claim employment expenses for working at home due to COVID-19 for income on this T4?", say YES to this question.
For more information, please follow these links
How do I claim home office expenses?
Am I eligible to claim employment expenses due to COVID-19?
