I am running an automated test to test a webform for my company. They have just installed a zipcode service which automatically adds the Street and City/Region when you fill in the Address and housenumber.
This autofill appears when you deselect the last form element (e.g. that of the housenumber). 
This is the order of the fields I'm using;

form:zipcode 
form:housenumber    
form:addition (optional) 
form:street (gets filled in by service after zipcode and housenumber are provided)
form:city (the other autofill field)

When you fill this form out manually, the address appears as soon as you click or tab into the addition field (as it is optional) but when it's done automated it doesn't work.
I have tried several things like; 

focus('form:addition') or
select('form:addition') but these don't work. I have tried 
type('\t') to tab to the form field, and 
type('form:addition', ' ') to type a space into the add. field and even 
type('form:addition', "") to leave it empty. None of these attempts have worked so far. 

Is there anyone that can help me with this?
Yesterday I found out that the zipcode service uses an Ajax call to retrieve the information. This call is executed when the zipcode and housenumber fields are both 'out of focus' (i.e. deselected).
The right statement I found to use this in my advantage is this;
selenium.fireEvent('form:number', 'blur') which deselects the last field where data was entered.
Selecting the next formfield with Selenium RC and python