Is it permissible to work in customer service for a company that is a subsidiary of Citibank and serves its customers?

We do not know the specific nature of your work, but if it means that you work for a company that serves "Citibank" customers by facilitating their transactions, then either your work involves facilitating usurious transactions or assisting in them through writing or promotion. This is impermissible because it constitutes participation in and assistance in spreading usury. Or, your work may not involve assisting in usury or participating in it, meaning the services you provide are outside the scope of usury. In this case, there is no objection to it, although it is preferable to seek other employment.