If you are self-employed and file your business income and expenses on Schedule C, you will enter the use of the room as a home office. To qualify, you must be self-employed and the room (or area) must be used regularly and exclusively for business. You can enter your home office expense by clicking on Search, typing in "Home Office" and then clickin on "Jump to home office".
For more information see the following TurboTax Help article: The Home Office Deduction