There's a box where you can ask them to take out a specific amount of money.
It's 4c), the last one.
Alternatively, you could add this extra income (estimated) on line 4a) on the W4 for your full time job, and not have the taxes taken out separately from this income, or just have some extra money taken out for taxes using 4c) on that form.
https://www.irs.gov/pub/irs-pdf/fw4.pdf
