Yes, depending on your income and age, you can do a spousal IRA.
The nice thing with TurboTax is you can go to Deductions and Credits section in TurboTax and down to Retirement contributions and enter it, and see the immediate impact.
And yes, as long as payment is made by April 15, 2019 and it is designated as a 2018 IRA contribution.
For more on IRA income limits see this link below and the 4th one down addresses spousal IRA's.
https://www.irs.gov/retirement-plans/retirement-plans-faqs-regarding-iras-contributions