I'm trying to use Role Delegation with prepopulate fields.

In order to set role 3 active, I can do the following in a hook_form_alter:
$form['account']['roles_change']['#default_value'][]=3

When I create a link to add a new user, such as the following, the role is not checked off:
admin/people/create?edit[account][roles_change][#default_value][]=3
I have even made sure that # is urlencoded to %23.

As a final test, printing the array shows in the hook_form_alter shows it came through exactly as expected:
dsm($_REQUEST['edit']);

Am I doing something wrong?