How do I get extension work in edge - replace selected text with another text

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Javascript

Hi,
I have a code that replaces a selected text by the mouse with another text
but I don't know how to get it worked.

my extension.js
_______________________________________

replaceSelectedText("HI");

function replaceSelectedText(replacementText) {
    var sel, range;
    if (window.getSelection) {
        sel = window.getSelection();
        if (sel.rangeCount) {
            range = sel.getRangeAt(0);
            range.deleteContents();
            range.insertNode(document.createTextNode(replacementText));
        }
    } else if (document.selection && document.selection.createRange) {
        range = document.selection.createRange();
        range.text = replacementText;
    }
}

_____________________________________
my Manifest.json

{
  "name": "Translator",
  "version": "1.0",
  "manifest_version": 2,
  "description": "This Extention helps you to translate",
  "author": "Ayman Ghandour",
  "browser_action": {
    "default_title": "Translator",
    "default_icon": {
      "38": "80.png"
    }
  },
  "permissions": [
    "

What I have tried:

I have tried to load the extension but when I select any text it's not changed to HI.
please help me ..

thanks a lot
Ayman