Conditional substitution in a URL string?

Is there a way to conditionally substitute a portion of a URL string? I need to insert one of two environment variables into this URL string in the {EH_DOMAIN} section depending on an input passed into the API.

I could just have an If-Then in the function stack, but then I have to maintain two similar but different versions of the API call configuration.

Is there a filter that will allow me to conditionally insert into the URL string based on an API input? I am aware of sprintf, but can it conditionally input different values based on an API input?

Thanks!