One of Schumer's Committee Assignments:
Committee on Rules and Administration, Ranking Member
The Rules Committee has jurisdiction over federal elections including the qualifications and credentials of Senators, contested elections, oversight of the Federal Election Commission and the Election Assistance Commission. The Committee oversees Senate procedures, rules, and buildings, and every four years, leads the planning of the Presidential Inauguration at the Capitol through the Joint Congressional Committee on Inaugural Ceremonies.
Just because Schumer isn't your Senator doesn't mean you can't write to him in his capacity of Ranking Member of this or any other appropriate committee. Address it to him by name, committee, position (chair, ranking member, member, etc.)
https://www.schumer.senate.gov/committee-assignments
