Repaying Retention bonus

I received a retention bonus for $14K but $6K taxes were taken out when it was paid to me, so I only received around $8K. Now, I'm paying back the $14K total in the same year. How can I recoup the $6K in taxes? What documents should I fill out or get from my ex-employer? I know very little about different tax situations and documents so would love help on it. Thanks!