It sounds like citibank is acting as the authorization server in this scenario.
You could look through the example apps we provide and pick the language you use. Then wherever we mention FusionAuth, you could instead slot in the citibank information.
If they support OIDC or SAML, you could also have FusionAuth delegate auth to them using by configuring citibank as an identity provider. A quick scan of the docs indicates they don't support those standards, though.
so my users can login
FusionAuth would be helpful in providing user management to your application and your users, outside of any interactions with citibank.
I recommend going through the 5 minute setup guide and kicking the tires yourself to get an understanding of what FusionAuth can do.
Cheers!