wc_pf_query_args

You can override query arguments via this filter hook

function wc_pf_query_args_custom_field($args, $form_data){

    //default search query
    $args['s'] = isset($form_data['keyword']) ? $form_data['keyword'] : '';

    return $args;

}
add_action('wc_pf_query_args', 'wc_pf_query_args_custom_field');

You can see the query parameter details here

https://developer.wordpress.org/reference/classes/wp_query/#taxonomy-parameters