where to add additional IRA contribution

To enter, edit or delete an IRA contribution -

Click on Federal Taxes (Personal using Home and Business)
Click on Deductions and Credits
Click on I'll choose what I work on (if shown)
Scroll down to Retirement and Investments
On Traditional and Roth IRA Contributions, click the start or update button

You can enter your traditional IRA contributions to either take a deduction, or track your basis if your income is too high to take a deduction. 

Here is how to do this in TurboTax Online:

1. Navigate to Federal > Deductions & Credits > Retirement and Investments > Traditional IRA and Roth IRA >  Start or Revisit
2. Choose Traditional IRA box and select Continue.
3. On the Is This a Repayment of a Retirement Distribution? screen, choose No.
4. On the Tell Us How Much You Contributed screen, enter the amount contributed and select Continue.
5. On the Did You Change Your Mind? screen, select No, then answer the questions on the following screens.

Here is some more information about IRAs: Deductions Allowed for Contributions to a Traditional IRA