First, I would wait until your return has been accepted. If it is rejected, you can go back and enter that Form 1099-R.
If it is accepted, then you will need to prepare an amendment to add that Form 1099-R.
The amendment forms will not be available until the Form 1040-X is finalized.
The IRS does not accept e-filed amendments so you would need to print out and send in that amendment.
The directions for filing an amendment will be here once the Form 1040-X is finalized.
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