Adding one default category and one from user by select option???

Adding one default category and one from user by select option??? – I want to add 2 categories, one is user selected from the select option and one is already selected by default. for example my by default categories ID is 20.

if(count($errors) == 0) {
			$allowed_tags = wp_kses_allowed_html( 'post' );
			$title = esc_attr($_POST['coupon_title']);
			$description = wp_kses($_POST['coupon_description'], $allowed_tags);
			$cat = (int)$_POST['coupon_cat'];
			$discount = esc_attr($_POST['coupon_discount']);
			$code = esc_attr($_POST['coupon_code']);
			$url = esc_url($_POST['coupon_url']);
			$date = $_POST['coupon_date'];
			if(is_user_logged_in() ) {
				$current_user = wp_get_current_user();
				$user_id = $current_user->ID;
			}
			else {
				$user_id = 1;
			}
			$post = array(
			'comment_status' => 'close', /* added by AB, open */
			'ping_status'    => 'close', /* added by AB, open */
			'post_author'    => $user_id,
			'post_content'   => $description,
			'post_status'    => 'publish', /* added by AB, pending */
			'post_title'     => $title,
			'post_type'      => 'coupons',
			);
			$post_id = wp_insert_post($post);
			if($post_id == 0) {
				$errors[] = __("The listing couldn't be added to the database, try again.", "Teo");
			}
			else {
				$cat = (array)$cat;

				wp_set_post_terms($post_id, $cat, 'category'); //setting the correct category