gistfile1.php

/**
 * add category field in ask form
 * @param  array $validate
 * @return void
 */
function ask_from_category_field($args, $editing){
    if(wp_count_terms('question_category') == 0)
        return $args;

    global $editing_post;

    if($editing){
        $category = get_the_terms( $editing_post->ID, 'question_category' );
        $catgeory = $category[0]->term_id;
    }

    $args['fields'][] = array(
        'name' => 'category',
        'label' => __('Category', 'categories_for_anspress'),
        'type'  => 'taxonomy_select',
        'value' => ( $editing ? $catgeory :  sanitize_text_field(@$_POST['category'] ))  ,
        'taxonomy' => 'question_category',
        'orderby' => ap_opt('form_category_orderby'),
        'desc' => __('Select a topic that best fits your question', 'categories_for_anspress'),
        'order' => 6
    );

    return $args;
}

add_action('ap_ask_form_fields', 'ask_from_category_field', 10, 2);

/**
 * add category in validation field
 * @param  array $fields
 * @return array
 * @since  1.0
 */
function ap_ask_fields_validation($args){

    if(wp_count_terms('question_category') == 0)
        return $args;

    $args['category'] = array(
        'sanitize' => array('only_int'),
        'validate' => array('required'),
    );

    return $args;
}
add_action('ap_ask_fields_validation', 'ap_ask_fields_validation');

/**
 * Things to do after creating or updating question
 * @param  int $post_id
 * @param  object $post
 * @return void
 * @since 1.0
 */
function after_new_question($post_id, $post)
{
    global $validate;

    if(empty($validate))
        return;

    $fields = $validate->get_sanitized_fields();

    if(isset($fields['category']))
        $category = wp_set_post_terms( $post_id, $fields['category'], 'question_category' );
    
}
add_action('ap_processed_new_question', 'after_new_question', 0, 2 );
add_action('ap_processed_update_question', 'after_new_question', 0, 2 );