If you have efiled your return you must wait until the IRS either accepts or rejects it starting on Jan. 28. If it is rejected you can add the form, fix the reason for rejection and resubmit the efile.
It it is accepted, you must wait until the IRS processes it and sends your refund. You can then begin an amended return form 1040X to add the form. Do not make any changes to your original tax return until you start the 1040X. It is estimated to be available on 2/7/19. The 1040X must be printed, signed, and you must mail it. It can't be efiled.