how can i create a pop up form?

How can i create a form that when one question is answered it pops up another form?

similar to the postcode search in the url provided

It is not possible to create a form with a popup condition.

However you can hide a field (or multiple fields) and show them based on a condition

Like here:

http://www.jotformpro.com/form/51225421689961

Heres the relevant section of the guide:

How to Show or Hide Fields Base on User's Answer