How can I focus on a selectize select box?

One way which appears to work is to call click() instead of focus():

$('.someclass input').click();

You should use:

selectized[0].selectize.focus();

$('.someclass input') returns a jquery object (wrapped set - see What does jquery $ actually return?). You don't want the wrapped set, you want the first element in the set that matches the selector.

Try $('.someclass input')[0].selectize.focus(); instead.