Can I make a contribution to my 401k until 4/15 for 2018? I already have my W2. Where do I list it to get credit for the contribution? Will this reduce my taxable income?

I want to make a contribution to my 401k to reduce my taxable income. I already have my W2.  If I am permitted to make that contribution up until 4/15, where is it reflected on my tax return? Will my employer have to file an amended W2?