You can add after filing; however, once you've filed your tax return do not make any changes until the IRS either accepts or rejects your return.
If accepted, wait until the IRS issues your refund (if applicable), then you can amend your return to add your renters' rebate. The following TurboTax FAQ can guide you when you are ready to amend: How do I amend my 2019 TurboTax Online return? - Community
If rejected, you can sign into your TurboTax Online account and fix what caused the rejection. At this time, you can also add the renters' rebate and re-file. Refer to the following FAQ for assistance: How do I fix a rejected return? - Community