Turn a Comma Separated string into multiple parameters

Summary: I have an API that takes 1 or more query parameters called base. These appear in the URL as ?base=GBP?base=USD?base=EUR for example.

I want my integration to take in a comma separated string, e.g. GBP,USD,EUR and pass these through to the API call.

How can I do this?

I have tried making a string "GBP?base=USD?base=EUR"

and passing that into a single instance of the base parameter but that errors.

I need to set it up like this :

Can that be done does anyone know?

Grateful for any helpful advice.