To enter an amount on Line 21 of IRS Schedule 1 (Form 1040) to reduce the taxable wage amount from Form 1040 Line 1 if that is appropriate in your situation:
When you print your return, the text See Attached will print to the left of Line 21 on Schedule 1 (Form 1040). An attached sheet will print with the return titled Other Income - Supporting Details for Schedule 1 (Form 1040), Line 21 which will print both the description and amount that was entered for Line 21.
Per the IRS website Claiming Tax Treaty Benefits:
If a tax treaty between the United States and your country provides an exemption from, or a reduced rate of, withholding for certain items of income, you should notify the payor of the income (the withholding agent) of your foreign status to claim the benefits of the treaty. Generally, you do this by filing Form W-8BEN, Certificate of Foreign Status of Beneficial Owner for United States Tax Withholding (PDF) with the withholding agent.
Per IRS Publication 519 U.S. Tax Guide for Aliens:
Page 48: Reporting Treaty Benefits Claimed
If you claim treaty benefits that override or modify any provision of the Internal Revenue Code, and by claiming these benefits your tax is, or might be, reduced, you must attach a fully completed Form 8833 to your tax return. See below, for the situations where you are not required to file Form 8833.
You must file a U.S. tax return and Form 8833 if you claim the following treaty benefits.
While these are the more common situations for which Form 8833 is required, see the publication for additional information.
Form 8833 is a fillable form which can be accessed and completed by following these steps:
Once the fillable form is completed, it will then become a part of the return and will be electronically filed and/or printed with the return.
Per IRS Publication 901, U.S. Tax Treaties, page 2:
This publication will tell you whether a tax treaty between the United States and a particular country offers a reduced rate of, or possibly a complete exemption from, U.S. income tax for residents of that particular country.
Tables in the back of this publication show the countries that have income tax treaties with the United States, the tax rates on different kinds of income, and the kinds of income that are exempt from tax.
CAUTION! You should use this publication only for quick reference. It is not a complete guide to all provisions of every income tax treaty.
See the IRS website Tax Treaties for additional information.