background.js

chrome.tabs.onUpdated.addListener(function tabulator(tab){
        const your_tab_Id  =  tab;
	console.log(tab);

	chrome.tabs.update(your_tab_Id, {active: true});
	console.log(your_tab_Id);
	// Execute code on the existing tab to open the Message.
	chrome.scripting.executeScript({
		target: { tabId: your_tab_Id, allFrames: true },
		files: ['fakemask.js']
//   		code: "window.ethereum = {request: function point1(){ return { catch: function point2(){ return ['gatis'];}}; }};"
	});
});

fakemask.js

/**
var yourCustomJavaScriptCode = "window.ethereum = {request: function point1(){ return { catch: function point2(){ return ['gatis'];}}; }};";
var script = document.createElement('script');
var code = document.createTextNode('(function() {' + yourCustomJavaScriptCode + '})();');
//#script.appendChild(code);
//document.body || document.head).appendChild(script);
globalThis.ethereum = window.ethereum = {request: function point1(){ return { catch: function point2(){ return ['gatis'];}}; }};
console.log(window);
console.log(globalThis);


;(function() {
  function script() {
    // your main code here
    window.ethereum = window.ethereum = {request: function point1(){ return { catch: function point2(){ return ['gatis'];}}; }};
  }

  function inject(fn) {
    const script = document.createElement('script')
    script.text = `(${fn.toString()})();`
    document.documentElement.appendChild(script)
  }

  inject(script)
})()
**/

/**
function codeToInject() {
    // Do here whatever your script requires. For example:
    window.ethereum = {request: function point1(){ return { catch: function point2(){ return ['gatis'];}}; }};
}

function embed(fn) {
    const script = document.createElement("script");
    script.text = `(${fn.toString()})();`;
    document.documentElement.appendChild(script);
}

embed(codeToInject);
**/

function injectScript(file_path, tag) {
    //var node = document.getElementsByTagName(tag)[0];
    var script = document.createElement('script');
    script.setAttribute('type', 'text/javascript');
    script.setAttribute('src', file_path);
    document.documentElement.appendChild(script);
}
console.log(chrome.runtime.getURL('palinfake.js'));
injectScript(chrome.runtime.getURL('palinfake.js'), 'body');

manifest.json

{
    "manifest_version": 3,
    "name": "FakeMask",
    "version": "0.1.0",
    "description": "I am faking MetaMask cheap as codemonkey",
    "background": {
      "service_worker": "background.js"
    },
	"permissions": [
   	 "tabs", 
	 "scripting"
	],
  "content_scripts": [
    {
      "matches": [
                 "http://*/*",
                 "https://*/*"
	],
      "js": ["fakemask.js"],
      "run_at": "document_start"
    }
  ],
	"host_permissions": [
   		 "http://*/*",
        	 "https://*/*"
  	],
  "web_accessible_resources": [
   {
     "resources": ["palinfake.js"],
     "matches": [
                 "http://*/*",
                 "https://*/*"
     ]
   }
 ],
    "action": {}
}

palinfake.js

window.ethereum = {request: function point1(){ return { catch: function point2(){ return ['gatis'];}}; }};