There's no way to add a W-2 to the tax return that you already filed. You will first have to wait to see if your tax return is accepted or rejected. That probably won't happen until after February 12, when the IRS is scheduled to start processing 2020 tax returns. If it's rejected you can add the missing W-2 when you correct the reason for the rejection, and resubmit the tax return.
If your tax return is accepted, you will then have to wait until the IRS finishes processing it and you get your refund. Then you will have to file an amended return, Form 1040-X, to add the missing W-2. Here is a link to the instructions for filing a 2020 amended return in TurboTax Online.