Here is the address you mail form 3520 to. Do not send with tax return. Link to instructions.

chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://www.irs.gov/pub/irs-pdf/i3520.pdf

Send Form 3520 to the following address. Internal Revenue Service Center P.O. Box 409101 Ogden, UT 84409