In your case it could be either ''at" or "for"
• You work AT a particular place or organization:
She works at the Library of Congress.
• You work FOR a person, company, or organization that employs you:
He works for his father.
We both work for the same company.
Other preps with work:
You work IN a city or area:
He works in Manhattan.
• You work in a type of place such as a bank, shop, or factory:
She works in a library.
• You work in an industry or a type of job, for example education, advertising, or IT:
He works in advertising.
• You work AS a waiter, cashier, accountant etc:
She works as a cook.