Predefine category for new products - this function will add a category automatically when the user creates a Product

functions.php

/* Predefine category for new products - this function will add a category automatically when the user creates a Product
*
* https://www.nuno-sarmento..com
*/

add_action( 'save_post', 'ns_auto_add_product_category', 50, 3 );
function ns_auto_add_product_category( $post_id, $post, $update ) {

    if ( $post->post_type != 'product') return; // Only products

    // If this is an autosave, our form has not been submitted, so we don't want to do anything.
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
        return $post_id;

    // Check the user's permissions.
    if ( ! current_user_can( 'edit_product', $post_id ) )
        return $post_id;

    $term_id = 69; // <== Your targeted product category term ID
    $taxonomy = 'product_cat'; // The taxonomy for Product category

    // If the product has not "69" category id and if "69" category exist
    if ( ! has_term( $term_id, 'product_cat', $post_id ) && term_exists( $term_id, $taxonomy ) )
        wp_set_post_terms( $post_id, $term_id, $taxonomy, true ); // we set this product category
}