Conditional redirecting

I want to redirect my form to 2 different pages dependent on their choice in a drop-down menu item (for example ‘yes’ or ‘no’). I already tried this:

if (document.getElementById('example').value=='yes') {
on_sent_ok: " location = 'http://www.redirectedpage1.com'; "
}
else {
on_sent_ok: " location = 'http://www.redirectedpage2.com'; "
}

I think the (document.getElementById(‘example’).value==’yes’) gives a Null value because I always get redirected to the second page.

Can anyone help me?

https://wordpress.org/plugins/contact-form-7/