How can I enter/add as an expense the $100+ turbo tax to my 2018 taxes?

[MichaelMc]

If, as most small businesses, you are a cash method taxpayer, and you purchased your TurboTax product prior to the end of 2018, I would simply include the cost in office supplies or software on your Schedule C.

If you made the purchase in early 2019, you will have to wait until you file your 2019 tax return in early 2020 to take the deduction.