I have filed and received my refund but now I would like to contribute to a roth ira for 2018. Is it too late? Do I just amend my return noting the amount?

[MichaelL1]

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.