I am making a catch up contribution to my 401k plan on April 15, 2018 for 2017. Where do I enter this in turbo tax? This amount will not be on my 2017 W-2.

[JohnW15]

Unfortunately, you can’t do this.  Elective deferrals for the 2017 tax year aren’t allowed after December 31, 2017. 

Employer-sponsored 401(k) plans are administered on a calendar-year basis, and contributions must be made within the calendar year.  Any contributions made after 2017 will apply to tax year 2018.