JavaScript works in console but not in my browser extension

Question

I am creating a Firefox Extension to change keyboard functions in the game https://agar.io/. The script.js below works fine if pasted in the console (CTRL+Shift+K) or loaded in scratchpad (Shift+F4) (scratchpad doesn't exists anymore in Firefox 72.0.2). But if I load my extension in Load Temporary Add-on in the about:debugging page, the functions

window.onkeydown({keyCode: 87});
window.onkeyup({keyCode: 87});
core.setMinimap(minimap); // This is a function of the game and works in the console for minimap=true.

don't work, as you can see by the logs test-points-finished that don't appear in the console when I press KEY_O or KEY_M.

So, why the functions above don't work inside the extension? Please explain me because I'm new to JavaScript.

script.js:

(function() {
    var minimap = false;

    var overwriting = function(evt) {
    if (evt.keyCode === 79) { // If reads KEY_O, then simulate press and release of KEY_W
        console.log("test-point-1-started");
        window.onkeydown({keyCode: 87}); // Simulate press of KEY_W
        console.log("test-point-1-finished");
        window.onkeyup({keyCode: 87}); // Simulate release of KEY_W
    }
    else if (evt.keyCode === 77) {// Toggle minimap with Key_M
        minimap=!minimap;
        console.log("test-point-2-started");
        core.setMinimap(minimap);
        console.log("test-point-2-finished");
    }
    }
    window.addEventListener('keydown', overwriting);
})();

Here is the manifest.json:

{

  "manifest_version": 2,
  "name": "My firefox extension",
  "version": "1.0",

  "description": "Test extension.",


  "content_scripts": [
    {
      "matches": ["*://*.agar.io/*"],
      "js": ["script.js"]
    }
  ]

}

Thanks.