First, you ask your employer for a refund for the taxes withheld in error. If you are unable to obtain a refund from your employer, you may file Form 843 - Application for Refund and Abatement. You must also attach the Statement for Refund of Social Security Tax withheld in error.

I have the same experience as you do. Also, based on my experience, if you cannot get a refund from the employer, do make sure ask them to provide a letter saying that there is an error for withholding FICA, and they are not willing to refund you. Attach that letter from your employer to the Form 843. In addition, make the payroll to correct the FICA withholding. Otherwise you will be ended up filing Form 843 for next year. Good luck!