How to bring back Post Categories in P2 v1.4

ajax.php

        // define $post_cat
        $post_cat = $_POST['post_format'];
        // and turn it into the category ID
        $post_cat = get_category_by_slug( $post_cat );	
	
        $post_id = wp_insert_post( array(
			'post_author'   => $user_id,
			'post_title'    => $post_title,
			'post_content'  => $post_content,
			'post_type'     => 'post',
			'tags_input'    => $tags,
			'post_status'   => 'publish',
            // bringing back P2 categories
            'post_category' => array( $post_cat->cat_ID )
            //
		) );