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Yes, Form 8606 is only used for IRAs and not for the mega backdoor.

You should have the distribution amount in 5a and $0 in 5b (rounded down) on Form 1040.

If you amend your return you will follow these steps:

1. Click "Federal Taxes" on the top and select "Wages & Income"
2. Click "I'll choose what to work on"
3. Scroll down and click "Start/Edit" next to "IRA, 401(k), Pension Plan (1099-R)" 
4. Enter the Form 1099-R (if not already entered)
5. On the "Did you move this money from a 401(k) to a Roth 401(k)?" screen answer "Yes"
6. On the "Did you make after-tax contributions to a 401(k), 403(b), or 457(b) plan?" answer "Yes" and the amount from box 5 should be listed in the field.

Please see How do I amend my federal tax return for a prior year? information.