Beginning in tax year 2019, Schedule 6 will be combined/included on the Form 1040 or the Form 1040-SR, whichever form you qualify to use. The foreign address will be included on page 1 of the 1040 or 1040-SR Form after the United States address. The Third-Party Designee will be on page 2 following line 24 on the 1040 Form or the 1040-SR Form.
For tax year 2018 you would still use Schedule 6 which is for the Foreign Address and Third-Party Designee. If entries are made on Schedule 6, the form would accompany Form 1040.
Before the Tax Cuts and Jobs Act reform was passed, Foreign Address was included on page 1 (after United States address) and Third-Party Designee followed line 79 on Form 1040.