The problem is here:

function NewListBox()
{
...
LISTBOX Test; // (the same as TWO did)
...

Variables defined in a function are local. They only exist within that function, so your global struct pointer still has no content.

You can either define a global LISTBOX resp. an array of global listboxes, or use mem_alloc for locally allocating global memory as suggested above. Mem_alloc is something for advanced programmers; the easiest way for beginners is a global array. You don't need to care about any pointers and memory allocation then.

LISTBOX listbox[100];