WPForms

functions.php

<?php
/**
 * WPForms automatically add category to posts created with post submission addon.
 *
 * @param int $post_id Post ID of the new post created
 * @param array $fields
 * @param array $field_data
 * @param int $entry_id
 */
function wpf_post_submissions_add_category( $post_id, $fields, $form_data, $entry_id ) {

	// Auto assign the category with the ID of 6
	wp_set_post_categories( $post_id, array( 6 ) );
}
add_action( 'wpforms_post_submissions_process', 'wpf_post_submissions_add_category', 10, 4 );