Where should I add the deduction from using an office in my home?

It goes with the form for the reason. If you have rentals, put it on sch E. If you have a business ,sch C. If you are an employee and your state allows the deduction, Form 2106 is used.

Are you self-employed?   If you are self-employed your home office goes on your Schedule C.   If you are a W-2 employee, there is nothing to enter about a home office for a federal deduction.

W-2 employees cannot deduct job-related expenses on a federal return.  Job-related expenses were eliminated as a federal deduction for W-2 employees by the tax laws that changed for 2018 and beyond.  Your state tax laws might be different in AL, AR, CA, HI, MN, NY or PA.

If you live in a state that lets you deduct job-related expenses, the information will flow from your federal return  to the state return, so enter it in Federal>Deductions and Credits>Employment Expenses>Job-Related Expenses