How to max 401k up to exact limit?

[Andrew S.]

Good morning,

So far this year, I have contributed $12k to my 401k.  I'd like to contribute up to the max $19,500.  I'm able to change my contribution percentage up to 75%. My pretax pay is $3912 per 2 weeks or $7825 per month. If I change my contribution to 75% (assuming it changes before Dec 15th) then I'll contribute an additional $5869.  Total of $17,869 for the year.  

Is there a way to do a "one-time" contribution?  I know it's complicated because 401k contributions are pre-tax. My 401k is through Fidelity. 
Thanks,

Andrew