This filter is used to enable the sending of the Item ID instead of Item Name when using the Fixed Costs or Dynamic Field Mapping settings.
add_filter( 'gform_freshbooks_send_item_id_for_fixed_dynamic', '__return_true' );
This hook has no parameters.
This code should be placed in the functions.php file of your active theme.
This filter is located in class-gf-freshbooks.php.