Vercel Rewrites for Serverless Sites

I’m building a static html css and js site, and I’m essentially using Vercel rewrites to do stuff like this:

getbrandkit.com/b/brandkit, should be given to the JavaScript as getbrandkit.com/b/main.html?b=brandkit. BUT, I never want the user to see the url parameters in their URL bar.

getbrandkit.com/b/brandkit is NOT an existent file in my code.

I’m using Vercel rewrites:

{
  "rewrites": [
    {
      "source": "/b/:b",
      "destination": "/b/main.html?b=:b"
    }
  ]
}

^^ This is clearly doing something because it is loading main.html when I visit getbrandkit.com/b/brandkit, but, I’m not able to get the b value using URL parameters, unless I physically type, getbrandkit.com/b/main.html?b=brandkit.

Ultimately, going to: getbrandkit.com/b/brandkit should console.log brandkit, and getbrandkit.com/b/anything should console.log anything.

TLDR: How do I get the value from getbrandkit.com/b/ANYTHING, on a serverless site, even though the file doesn’t exist.