Where to find where to enter allowable expenses for an in-home rented room with shared spaces of an AirBNB short-term rental hosting business?

It depends. You can select that option (vacation rental) to enter your rental income and expenses. TurboTax will  help and guide you with number of days rented and square feet of rental area vs total square feet of the home. Read the screens carefully to ensure you enter the appropriate information for your vacation short term rental.

• 10 Tips for Airbnb, etc Vacation Rentals
• Where do I enter income and expenses from a rental property?