There are a number of ways of doing this i.e. server side and client side. The suggestion below is client side using Javascript, this way you won't need a server to test it out, although you will for the second of your queries(some on to that later).

Right number 1: validating the form. You want to generate a pop up window when a user has not filled in a particular field. This is Javascript.

Here's what you do.
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