How can I find and access a JavaScript property ('Blazor') of 'window' with a (GreaseMonkey) user script?

Question

With the following (GreaseMonkey) user script I am trying to find (and later access) a JavaScript object (namely 'Blazor'):

// ==UserScript==
// @name         Test
// @version      1
// @grant        none
// @run-at       document-end
// ==/UserScript==

window.addEventListener ("load", doStuff);

function doStuff() { 
    setTimeout(function() {
        console.log("window.hasProperty('Blazor'): " + window.hasOwnProperty("Blazor"));
        console.log("'Blazor' in window:           " + ('Blazor' in window));
        console.log(window);
    }, 2000);    
}

When I run the script (i.e., reload the page), I get the following output in Firefox' console:

So window.Blazor can not be found. However, when I inspect (click on) the window object I can see the Blazor property:

Calling 'Blazor' in window in the console also returns true.

Why is this property of window not (yet?) available and how can I find and access it?

Update:

Based on madalinivascu's answer I tried the following:

function doStuff() {
  console.log("<doStuff>");
  
  var promise = new Promise(function(resolve, reject) {
    var interval = setInterval(function() {
      console.log("checking for Blazor");
      if ('Blazor' in window) {
        console.log("Blazor found");
        clearInterval(interval);
        resolve(window.Blazor);
      }
    }, 1000);
  });
  
  promise.then(successCallback, failureCallback);
  
  console.log("</doStuff>");
}

function failureCallback(error) {
  console.log("failure: " + error);
}

function successCallback(result) {
  console.log("success: " + result);
}

which results in the following output:

So the function checking for Blazor is called indefinitely without success in finding it.

Answer 1

The solution is to use script injection, since the GreaseMonkey script runs in another context and thus gets another window object.

More precisely, attaching the function doStuff with addJS_Node defined in that answer (addJS_Node (null, null, doStuff);) instead of using window.addEventListener ("load", doStuff); works.