If you have already filed the 2023 tax return then you will have to amend the return to add a dependent.
You will have to wait for the IRS to either Accept or Reject your filed federal tax return.
If the return is rejected you can make the necessary changes and e-file again.
If the return is accepted you will have to amend your original tax return. An amended return, Form 1040-X, can only be printed and mailed to the IRS if the original tax return was not e-filed. The IRS will take up to 20 weeks or longer to process an amended tax return.
Before starting to amend the tax return, wait for the tax refund to be received or the taxes due to be paid and processed by the IRS.
See this TurboTax support FAQ for amending a tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-change-correct-return-already-filed/L4VjJ9BA2_US_en_US?uid=ld1n99kr
If you have not filed the 2023 tax return -
Click on My Info on the left side of the online program screen. Add a dependent.