Call background function of Chrome extension from a site

I am looking for a function inside a webpage te activate a chrome extension.

Imagine that http://www.example.com/test.html contains:

<script>
hello();
</script>

And my background page contains the definition of the hello function:

function hello() {
    alert("test");
}

How can I make sure that the Chrome extension's background page's hello is called when test.html calls hello();?