Requestly doesn't provide additional filters in Script Injection to detect whether you are injecting in Parent Page or Iframe.

However, there's one trick you can do.

You can additional checks in your script so that It only executes inside Iframe something like this

if (window.self !== window.top) {
    // Your script code goes here
}

However, If you can define your URL Pattern such that It only matches the Iframe then it's even better. Then you don't have to add this check but do better URL targeting such that Script Injection only works with Iframe URL and not parent URL.