If you for example submit a form using POST and not GET, the URL will not change. How can you create and send a virtual pageview to BTB when the form is submitted successfully so this visitor can be bucketed as "converted"?
BTBuckets provides a Javascript API with useful functions. On this page you're going to find the available functions and how to use them.
What you're looking for is the "$BTB.trackEvent" function, equivalent to the trackPageview of Google Analytics. The usage is similar, configure an Event on your bucket profile, and use the same value on the function. For example, if you configure an event named "form_submitted", just call the function on the page like this: