Unfortunately, you can't do this.
401(k) elective deferrals needed to have been made through payroll deduction by the end of the tax year, and there's no grace period after that to make additional contributions.
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 a tax year ends will apply to the tax year in which they’re made.
[Edited 3|17|2020 8:20 am]