Javascript to set default value on popups doesn't work

[Marcela Junyent]

I want to apply a javascript to set a default current year value. I use the following script:

<script type="text/javascript">
CurrentYear= document.getElementById('customfield_10420');
CurrentYear.readOnly = true;
var year = new Date();
CurrentYear.value = year.getFullYear();
</script>

it works when the create window is not as popup.But if the screen is opened as popup, the script is not executed.

someone knows haoy to fix this? Thanks