Form 8843 Statement for Exempt Individuals and Individuals with a Medical Condition, is used to exclude days of presence in the U.S. for certain individuals. According to the instructions for the form, it would either be mailed to the IRS alone or with Form 1040-NR U.S. Nonresident Alien Income Tax Return.
Per IRS Publication 519 U.S. Tax Guide for Aliens, on page 6:
Form 8843. If you exclude days of presence in the United States because you fall into any of the following categories, you must file a fully completed Form 8843.
Attach Form 8843 to your 2020 income tax return. If you do not have to file a return, send Form 8843 to the following address.
Department of the Treasury
Internal Revenue Service Center
Austin, TX 73301-0215
You must file Form 8843 by the due date for filing Form 1040-NR. The due date for filing is discussed in chapter 7.
To enter information for Form 8843:
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.