Adding a custom input field when creating a new Order

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As the name if this topic suggets I would like to add a custom field to store some additional data when I click the new order. - please excuse my drawing skills. 

How would I go about achieving this? I have a programming background so if an app needs to be created I don't mind it. This should be a completely trivial task but I have been searching for an hour now and I didn't find anything useful, any help would be greatly appreciated. The field also doesn't need to show onto the front-end. 

EDIT: is it possible to load a javascript file and introduce it into the back-end of Shopify? I feel like this would be an easiest solution, if it's possible. I would just dynamically change the contents of the page based on URL.