How do I claim box 12 k as pretax vision and dental

You do not claim this as pretax vision and dental.

Box 12 code K –  is 20% excise tax on excess golden parachute payments. Include this tax on Form 1040 Schedule 2, line 17k, enter the total amount of the tax.

I think you mean Box 14 code K which is for the pre-tax dental and vision insurance deduction amount.  This amount  was withheld during the year to pay for your dental and vision coverage.  It is a reporting number on the W-2 and not used in calculating taxable wages.  

The amount reported with code K has already reduced the taxable income reported in box 1 taxable wages of the W2 form, so you should not be using this amount as a part of your qualifying medical expenses for the tax year (if you are itemizing deductions).