You are correct, any money you are paid or were paid on your behalf is reportable income. The proper way would have been for the payer to include it on your 1099-Misc. If not, you'll be given a chance to enter it separately in TT. After entering your 1099-Misc, you'll be asked if you received other money.
Yes, it is 2018 income, not 2017, since that's when your received it. Even though the check was made out to the IRS, you had constructive receipt, when it was submitted with your tax return.
