The Autofill module makes it possible to autofill the values from one textfield to another while typing. Once your field input was customized the autofill functionality will be detached.

Install the Autofill module as you would normally install a contributed Drupal module. Visit https://www.drupal.org/node/1897420 for further information.

1. Open the "Manage form display" configuration for your designated content type.
2. Click on the widget settings icon for the field that should be autofilled.
3. Enable "Autofill from another field"
4. Select the source field from which the field should be autofilled.

• Drupal core