Welcome to the Auth0 Community!
We appreciate you reporting this issue to us! Indeed, the same behaviour is met on our end as well, which seems to be a UI issue/bug → The " users " should be an upload form, but currently it a text field.
We have notified our Product team of this issue and they will be looking into this as soon as possible.
While we do not have an ETA for the resolution of this issue, the API can still be used with other tools such as Postman and the functionality should remain the same. Also, pasting the payload into the field is possible and it will expand the field for better visibility.
As an alternative, I do recommend using the User Import / Export Extension which offers the same functionality of importing users via an uploaded JSON file. More information on the file type and schema can be seen in the following documentation on Bulk User Import Database Schema and Examples, in case this comes in handy.
Thank you for your diligence and for reporting this issue!
Have a great day forward!
Gerald