Hello - my employer withheld excess FICA taxes due to them using Pay Agents. Where do I mail form 843 along with all supporting documentation?

You mail the form and documentation to the IRS service center for the state where you live.  Use the address for a Form 1040 without payment - https://www.irs.gov/filing/where-to-file-paper-tax-returns-with-or-without-a-payment