WordPress CMB2 – Run function on save

If you haven’t gotten the answer to this yet, I found the answer by sifting through the source code. I was able to hook into the post save event by using: cmb2_save_{$object_type}_fields_{$cmb_id}. This hook fires whenever you save a certain object type with a specific metabox ID.

So, in your case your $cmb_id is test_metabox, and $object_type will be page.

Hooking in would look something like this:

add_action( 'cmb2_save_page_fields_test_metabox', 'my_post_save_function', 10, 3 );

 function my_post_save_function( string $object_id, array $updated, CMB2 $cmb )
 {
     // code
 }

The docblock in the source is messed up, I corrected the parameter types above: Source