it depends. According to irs.gov, form 843 is used to claim a refund or request an abatement of certain taxes, interest, penalties, fees, and additions to tax. This is not be be used to claim excess tax withholdings.
Instead, the correct procedure is to do as Hal suggested above and first request that your employer issue a corrected W2 correcting the mistake that you think may have occurred. the Irs will then send a demand letter to your employer to furnish you a corrected W2. If this isn't done by the end of February, then the IRS will send you instructions on filling out form 4852. Complete instructions are found here.
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