custom field only for particular category

Hi,
it is possible but only with the Custom Fields extension and the Preselect Ad Category snippet https://github.com/simpliko/wpadverts-snippets/blob/master/preselect-ad-category/preselect-ad-category.php

With this setup you would have a different form scheme (with an additional field) for one of categories and when user will access a page with [adverts_add] shortcode he would need to first select the category and then he will see a correct [adverts_add] form.