How to autofill a form field with random number using javascript in google chrome extension?

In the options of my chrome extension I'd like to autofill a form field when the options page is opened. I have some code for this in the option.js file but it doesn't seem to autofill on opening.

I've tried some code which I will attach.

function() {
        var randomString = function(length) {

            var text = "";

            var possible = "0123456789";

            for(var i = 0; i < length; i++) {

                text += possible.charAt(Math.floor(Math.random() * possible.length));

            }

            return text;
        }

        // random string length
        var random = randomString(10);

        // insert random string to the field
        var elem = document.getElementById("email").value = random;

    }();

I'm using the 'email' form field for the moment to test the code - I would create a new form field once the code works.