How to add existing categories into a post using wp_insert_post

I tried to add a category based on a post as seen on this link:
How we add new categories by wp_insert_post

Here, I added ‘post_category’ and default category IDs in an array as shown below:

$my_post = array(
    'post_title'    => 'test',
    'post_content'  => 'hello',
    'post_status'   => 'publish',
    'post_author'   =>  1,
    'post_type'     => 'product',
    'post_category' => array(2,3,4,5)
 );
 $post_id = wp_insert_post($my_post);

All of the fields above have been added, except for the post_category. I wanted to add more than one category per product, and what I am using is an array.

What is probably the problem here?