add web app manifest to another persons website

Question

Is it possible to inject a web app manifest on another persons website? I have tried running javascript that injects the manifest but it doesn't seem to work.

var myDynamicManifest = {
  "name": "Site",
  "short_name": "Site",
  "description": "Description",
  "background_color": "#000000",
  "theme_color": "#0f4a73",
}
const stringManifest = JSON.stringify(myDynamicManifest);
const blob = new Blob([stringManifest], {type: 'application/json'});
const manifestURL = URL.createObjectURL(blob);

var link = document.createElement("link");
link.setAttribute("rel", "manifest");
link.setAttribute("id", "my-manifest-placeholder");
document.head.appendChild(link); 

document.querySelector('#my-manifest-placeholder').setAttribute('href', manifestURL);

completion(true);