Your former employer should take care of the FICA part after you repay them. For federal income tax, you'll claim a tax credit when you file your return according to "Repayment over $3,000" under IRS Pub 525. State income tax may have a similar process, though it can vary depending on the state.
https://www.irs.gov/publications/p525#en_US_2022_publink1000...