Injecting a JS code into all pages of a website

I'm trying to inject a JS code into a website, where the code gets executed on all of its pages. I found out that this could be done using the chrome content scripts. Therefore, I've done the following:

1. I've created "contentscript.js".

   var x = [] s.src = 'AN EXTERNAL JS FILE'; (document.getElementsByTagName('head')[0] || document.body).appendChild(s);

2. I've created "manifest.json".

   { "name": "Sample", "version": "1.0", "manifest_version": 2, "permissions": [ "tabs", ":///*" ], "icons": { "128":"logo.png" }, "content_scripts": [{ "js": ["contentscript.js"], "matches": [""] }] }

3. Both files exist in the same folder.

4. I've loaded them into the chrome extensions.
5. The script gets executed as expected.

The script I'm injecting loads an external JS file into the page and places a script element under the head tag. This JS also uses the "x" variable I defined in the injected JS script. But the problem that it can't read it. In the console, it throws an error that the "x" variable is not defined.

How can I define the variable in a way it makes it accessible to the external JS file, mentioning that I don't have access neither to the website nor to the external JS file?

Thanks,