Custom Input Fields are built using WebComponents. 

This checklist should get you started to have your first custom input field, like in this example. 

1. In your App-Folder, create a folder called system (if it doesn't exist yet)
2. In the system folder, create a folder field-[type]-[subtype]
3. In this field-folder, create an index.js with this content - this will be the primary file loaded when someone uses this field
4. Verify that you can now see this field in the field-type picker