so is it possible to define a folder outside the wp-$folders ?

i created a html page and i want to use WordPress hooks and filters in it simply the WordPress PHP syntax

im still learning php – and i dont know what file should i call to make it work

my wordpress file system looks like

wordpress 
|
|
|_ helloworld _ index.php ( contain php code + html - js - css )
|_ wp-admin
|_ wp-content
|_ wp-includes
normal WordPress file system
..
.

i want the index.php that inside the hello word (dir) to make it able to run WordPress commands and not be just static page

Note : i dont want to turn the html into a template (: