This is a great answer from Super User Carl:
In the Business Assets section, check first that there's nothing in that list already for your startup costs. Assuming there's not, start entering your new asset (for your startup costs)
Click the Add An Asset button, select Intangibles, Other Property and continue.
Select Amortizable Intangibles, continue.
label it "Start-Up Costs", enter your entire amount, and the date needs to be the date your business opened, or one day after if fine. Continue.
Purchased new, and yes, used 100% for business. Continue.
Code section is 195: Business Startup Cost. Continue.
useful life is 15 years. (Timeframe specified/mandated by IRS for startup costs) Continue.
Look at the details if desired. Click continue and that does it.
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"