Deductions & credits

Hi, I'm still confused as to how this is administered to the 401k provider. In my case, I changed employers and the 401k with the former (fidelity) was distributed. $12k and I kept it as a covid distribution to be taxed over 3 years. I am with a new employer and new 401k provider (schwab). How do I 'pay it back'?  If I simply withhold the funds from my check it goes into pre-tax as current year contributions to the 401k.. If I report part of that on my 8915-F as paid back, I would also have to reduce the amount in 12b code D on my W-2, I believe. This doesn't feel right, so I'm still not sure how to handle still.