, . , , .
, drupal ( js Drupal standard js) , . , "ad-hoc" jQuery.
However, the easy part !;) The difficulty is that the functionality is achieved through AJAX, and not just by creating an additional client part of the form field. This is due to the fact that the API form must completely regenerate the form, otherwise - upon submission - additional fields that were not sent initially to the browser will simply be ignored (this is intended to prevent injection).
Hth at least a little bit! Sorry for being so concise.
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