TurboTax Online does not give you a way to enter line 18 "as previously adjusted by the IRS".
This is a serious deficiency.
( You could call TurboTax Support to find out if this is a true statement or there is a trick to make it happen.
I don't think so.)
After going through the amendment to get your new or changed forms and schedules,
refer to the IRS Forms page at www.irs.gov.
The blank form 1040-X in fillable PDF format and its instructions are available there.
Refer to the Instructions for what is to be attached to Form 1040-X.
Do not include your old 1040 nor your revised 1040 because the Form 1040-X reflects any changes there and becomes your new tax return.