functions.php

<?php
/**
 * Filter AnsPress question permastruct.
 *
 * @param array $structure Question rules.
 * @return array
 */
function my_ap_question_perm_structure( $structure ) {
	add_rewrite_tag( '%question_category%', '([^/]+)' );
	$question_slug = ap_get_page_slug( 'question' );
	$structure['rule'] = ap_base_page_slug() . '' . $question_slug . '/%question_category%/%question%';
	return $structure;
}
add_filter( 'ap_question_perm_structure', 'my_ap_question_perm_structure' );

/**
 * Filter question link and replace %question_category% with first category slug.
 *
 * @param string $link
 * @param WP_Post $post
 * @return void
 */
function my_ap_question_post_type_link( $link, $post ) {
	$cat = get_the_terms( $post->ID, 'question_category' )[0];
	$link = str_replace( '%question_category%', $cat->slug, $link );

	return $link;
}
add_filter( 'ap_question_post_type_link', 'my_ap_question_post_type_link', 10, 2 );