Can Railgun be installed only within a sub-domain or a specific path?

I wish to install Railgun specifically on our AWS ELB (load balanced stack) that’s just our REST API.

Our front-end is static assets at so it does not need Railgun.

Our API is at - and that’s where we specifically wish to install it.

Note that my Cloudflare account encompasses the entire domain and all sub-domains - numerous ones, some of which are even hosted by third-parties outside our control.

How would I go about installing Railgun just for the API endpoint, which is on an entirely different AWS stack to the other endpoints?