Calling external API from JavaScript in Dynamics Portal through custom code in CRM

We are trying to call an external API from Javascript in one of the pages of the Dynamics Portal. Unfortunately this external API is only available from a fixed IP address. So we need some custom server code in Dynamics CRM that calls the API and is available from Javascript.

Any suggestions what architecture we can select best?

Hope this question is clear to you all. Many thanks for your responses!

(The external API provides us with a list of addresses that belong to a given postal code.)