Per IRS Instructions for Form 1040 and 1040-SR, on page 100:

Line 10

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.

• A refund of tax, other than a tax for which a different form must be used. (See Do not use Form 843 when you must use a different tax form next.)
• An abatement of tax, other than income, estate, or gift tax. Employers cannot use Form 843 to request an abatement of FICA tax, RRTA tax, or income tax withholding.
• A refund to an employee of excess social security, Medicare, or RRTA tax withheld by any one employer, but only if your employer will not adjust the overcollection. See the instructions for line 7.
• A refund to an employee of social security or Medicare taxes that were withheld in error, but only if your employer will not adjust the overcollection. See the instructions for line 7. If you are a nonresident alien, see Pub. 519, U.S. Tax Guide for Aliens, for specific instructions.
• A refund of excess tier 2 RRTA tax when you had more than one railroad employer for the year and your total tier 2 RRTA tax withheld or paid for the year was more than the tier 2 limit. See the instructions for line 3.
• A refund or abatement of interest, penalties, or additions to tax, caused by certain IRS errors or delays, or certain erroneous written advice from the IRS.
• A refund or abatement of a penalty or addition to tax due to reasonable cause or other reason (other than erroneous written advice provided by the IRS) allowed under the law.
• A refund of the penalty imposed under section 6715 for misuse of dyed fuel.
• A refund or abatement of tier 1 RRTA tax for an employee representative.
• A refund of a branded prescription drug fee.

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.

• The amount, if any, the employer has repaid or reimbursed you for excess taxes withheld.
• The amount, if any, of credit or refund claimed by the employer or authorized by you to be claimed by the employer. The employer should include in the statement the fact that it is made in support of your claim for refund of employee tax paid by the employer to the IRS.

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 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.