At the beginning of this year, I had a 401k automatically roll over into an IRA. Can I contribute to this IRA before April 15, 2019 and have the credit on my 2018 taxes?

Do you have earned income in 2018?
W-2 income or other other income subject to Self-employment tax?

Yes. I have earned income

Ok, then the best and easiest way is to go to:

Federal Taxes

Deductions and Credits

I'll chose what I work on

scroll down to Retirements and Investments

then the first one is Traditional and Roth IRA contributions.

You can enter what you are thinking about contributing and see the tax impact (if a Traditional IRA)

Then you just need to make the contribution to the IRA by April 15, 2019 and the contribution must be designated as a 2018 contribution.