Customer fields scope is defined in the field configuration. Unfortunately, it's not possible at the moment to change the field configuration programatically using REST APIs.
There was a feature request raised in the past to Allow configuration of custom field options via REST API, but it was closed due to other large number of JIRA feature suggestions.
Not that I am aware of. JSON web tokens are for authentication your apps. Once authenticated, you will have to use the available REST API endpoints to perform various tasks.
So, in this case, the endpoint to edit field configuration is not available at the moment.
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