Chrome Extension get selected text

You can use the last expression evaluated by the executed code in a callback function:

chrome.tabs.executeScript( {
  code: "window.getSelection().toString();"
}, function(selection) {
  document.getElementById("output").value = selection[0];
});

You can do this by using Extensions Messaging. Basically, your "background page" will send the request to your service. For example, lets say you have a "popup" and once you click on it, it will do a "Google search" which is your service.

chrome.extension.onRequest.addListener(function(request, sender, sendResponse) {
    if (request.method == "getSelection")
      sendResponse({data: window.getSelection().toString()});
    else
      sendResponse({}); // snub them.
});

Some References

1. Creating a chrome extension which takes highlighted text on the page and inserts it into a textarea in popup.html

Or you can use this plugin

2. https://chrome.google.com/webstore/detail/view-selection-source/fbhgckgfljgjkkfngcoeajbgndkeoaaj

For Angular 8.2 I use this code:

chrome.tabs.executeScript( { code: 'window.getSelection().toString();'}, selectedText => {
 (document.getElementById('text-input') as HTMLInputElement).value = selectedText[0];
  console.log(selectedText[0]);
});