How do I use the value of a webform_field as the default-value for a later webform_field? Is there a way with Drupal 8 Webform UI to set the default value of a text field as the value of an earlier text field in the same form, and update via Ajax?

example: 

user_name:
     '#type': textfield
     '#title': 'User Name'

nick_name:
    '#type': textfield
    '#title': 'Nick Name'
    '#default_value': '[webform_submission:values:user_name]'
    '#ajax': true


But the default value does not render the token and just prints plain text.

Any help is much appreciated, thanks.
The short answer is 'No' to using tokens to sync two element values, unless the two elements are on different pages. The simplest solution is a write a little JavaScript to sync the nickname with the username.

You can search for examples of sync'ing two input fields. 

The Webform module's Options admin widget uses a little JS to sync an option's value with the option's text. @see webform.element.options.admin.js