How to add a password field to an entity as the Field_UI has none to offer
Issue:
You want to add a password field to an entity via the field_ui. It does not work, and – as a reminder to recurring devs – is not supposed to work. The UI is switched off in the core definition.
Solution:
If you need a password field, first evaluate the necessity to save a password in the database – this might create a serious security risk.
If your project requires it anyway, you either have to create a custom widget for a textfield or use the module Password field.