You have until 4-15-2019 to do an IRA, just make sure it is designated as a 2018 tax year contribution.
The file an Amended return to note the contribution (which will not changes the tax) but will allow you to track the basis in the Roth.
I would actually enter it in the TurboTax program first to make sure you can do a Roth, prior to actually depositing it in the IRA.