The intended solution

The initial intention of this project was to create an add-on for Ninja Forms. However, anyone who looks at the Ninja Forms API and documentation for more then 15 minutes will realize how lacking the information actually is. If you continue and try to implement any of the examples you will find out that the code actually doesn't work.

After much pain a suffering I found some posts and documentation scattered on the internet from other devs experiencing the same problems and finally put together a solution that works.

The actual solution

Adding code to the WordPress functions.php file is the simplest and most efficient way to add a WordPress hook or modification to Ninja Forms. Creating a Ninja Forms add-on is actually creating a stand alone WordPress plugin that is linked or to Ninja Forms. The add-on isn't necessary unless you intend to tie into WordPress Admin settings and menus.

For my purposes this was a one off project so I opted for the functions.php solution.