Modifying an existing form on a drupal 8 site
These are notes/documentation on how to change an existing form on a drupal 8 (D8) site so that you can add an element to it. "[module]" needs to be replaced with the module name in the areas it is referenced in this document.
Create a D8 module
You need to create a drupal module with at least 2 files (with appropriate content for them to be an actual D8 module):