Per IRS Instructions for Form 1040 and 1040-SR, on page 100:
Excess Social Security and Tier 1 RRTA Tax Withheld
If you, or your spouse if filing a joint return, had more than one employer for 2020 and total wages of more than $137,700, too much social security or tier 1 railroad retirement (RRTA) tax may have been withheld. You can take a credit on this line for the amount withheld in excess of $8,537.40. But if any one employer withheld more than $8,537.40, you can't claim the excess on your return. The employer should adjust the tax for you. If the employer doesn't adjust the overcollection, you can file a claim for refund using Form 843. Figure this amount separately for you and your spouse.
You can't claim a refund for excess tier 2 RRTA tax on Form 1040, 1040-SR, or 1040-NR. Instead, use Form 843.
TaxAct® automatically calculates any excess social security tax withheld from your wages and reports the excess on Line 10 of Schedule 3 (Form 1040) Additional Credits and Payments.
Per IRS Instructions for Form 843 Claim for Refund and Request for Abatement, on page 1:
Use Form 843 to claim or request the following.
On page 5:
Refund of excess social security, Medicare, or RRTA tax. If you are claiming a refund of excess social security, Medicare, or RRTA tax withheld by one employer, you must, if possible, attach a statement from the employer. The statement should indicate the following.
If you cannot obtain a statement from the employer, you should attach a statement with the same information to the best of your knowledge and belief and include in the statement an explanation of why you could not obtain a statement from the employer. Attach a copy of your Form W-2 to prove the amount of social security, Medicare, or RRTA tax withheld.
For more information, see the instructions for Form 843.
To view the worksheet calculation in TaxAct for excess FICA withholding:
Note. If you see a message indicating you have not paid your return fees, click the View/Pay your return fees link, which will direct you through the Paper Filing steps. Continue through the screens until you have processed the payment for your product fees and then repeat the steps to print.
Navigate to the worksheet using the Forms list, then click the pink Printer icon once the worksheet is opened on your screen.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.