Sending message to background script

You cannot simply use messaging the same way you would use it in a content script from an arbitrary webpage's code.

There are two guides available in the documentation for communicating with webpages, which correspond to two approaches: (externally_connectable) (custom events with a content script)

Suppose you want to allow http://example.com to send a message to your extension.

1. You need to specifically whitelist that site in the manifest:

     "externally_connectable" : {
       matches: [ "http://example.com" ]
     },
   

2. You need to obtain a permanent extension ID. Suppose the resulting ID is abcdefghijklmnoabcdefhijklmnoabc

3. Your webpage needs to check it's allowed to send a message, then send it using the pre-defined ID:

   // Website code
   // This will only be true if some extension allowed the page to connect
   if(chrome && chrome.runtime && chrome.runtime.sendMessage) {
     chrome.runtime.sendMessage(
       "abcdefghijklmnoabcdefhijklmnoabc",
       {greeting: "yes"},
       onAccessApproved
     );
   }
   

4. Your extension needs to listen to external messages and probably also check their origin:

   // Extension's background code
   chrome.runtime.onMessageExternal.addListener(
     function(request, sender, sendResponse) {
       if(!validate(request.sender)) // Check the URL with a custom function
         return;
       /* do work */
     }
   );