I am automating testing of a web page in which there exists two multi select fields on a form. The one on the left contains a list of country names, the one on the right contains the ones you have selected. Countries can exist on one list but not the other. When you click a country it moves to the other list (you get the idea). When the form is submitted the values in the POST look like this (from chrome inspection):

data[Campaign][unselected_country_codes]:
data[Campaign][unselected_country_codes][]:AF
data[Campaign][unselected_country_codes][]:AX
data[Campaign][unselected_country_codes][]:AL
data[Campaign][unselected_country_codes][]:DZ
data[Campaign][unselected_country_codes][]:AS
data[Campaign][unselected_country_codes][]:AD
data[Campaign][unselected_country_codes][]:AO
data[Campaign][unselected_country_codes][]:AI
data[Campaign][unselected_country_codes][]:AG
data[Campaign][unselected_country_codes][]:AR
... (ALL countries not selected are in this list)

data[Campaign][country_codes]:
data[Campaign][country_codes][]:US
data[Campaign][country_codes][]:AQ
data[Campaign][country_codes][]:BD
... (this part contains the ones we have selected)

and here is what the HTML looks like for the select list of the countries you want included:

<select name="data[Campaign][country_codes][]" multiple="multiple" id="CampaignCountryCodes" data-quicklist-ref="CampaignCountryCodes_quick" data-alter-ref="CampaignCountryCodes_alter">
<option value="US" selected="selected">United States</option>
<option value="AQ" selected="selected">Antarctica</option>
<option value="BD" selected="selected">Bangladesh</option>
</select>

What I don't understand is, how would I set those form values at the time of testing? I don't really understand the <select> attribute of a form comes through as an array of values or what? I would like to be able to submit the form and have a list of countries that are on, and a list that are off.

Here is an example of what I do to set other values on the form that works correctly. I really have no idea how to do this for a multiple and that is the question being asked.

This example is a text area that works just great.

casper.waitForSelector(x('//*[@id="some text area"]'),
   function success() {
        test.assertExists(x('//*[@id="some text area"]'));

        this.fill('form#campaign_form', { 
            'data[Campaign][some_field]': 'include',
            'data[Campaign][the_field]': myvar + "\n"
        }, true); 
   },
   function fail() { ... }
);