Adding custom field to REST taxonomy

Hello Everyone
I’m trying to add a custom field to the response of
wp-json/wc/v3/products/attributes/3/terms

i don’t know what $object_type i should assign to register_route_field to get the response I want
my product taxonomy is: pa_multicolor

register_rest_field(
        array(  'product_attributes', 'product_attribute_pa_multicolor', 'terms', 'term', 'pa_multicolor', 'product_attribute'  ),     
        'acf_custom',           
        array(
            'get_callback' => 'ground_get_cb',
            'update_callback' => 'ground_update_cb'
        )
    );

Both the callbacks return a string for now.

‘acf_custom’ doesn’t appear in the JSON of the reponse

I’ve googled and searched the forum but couln’t find an answer