How to add js file in google chrome extension's background script, when no HTML file is available?

I need to add this JavaScript file to background.js file without background.htm file.

this is my background.htm file.

<script src="https://www.testme.com/test/3.0/testapp.js"></script>

Everything works perfectly when I tested in a test project with background.htm file.

But my production project has no background.htm, and only background.js file is exist. How can I add this testapp.js file reference directly to background.js file?