Thanks Dave!
it works.
1. set subsite_url [lindex [site_node::get_url_from_object_id -object_id $subsite_id] 0]
2. set subsite_url [export_vars -base "$subsite_url" {project_item_id}]
3. It is a few strange how the link looks like.
/project-manager/fred-dispatch-development/?project_item_id=2644
i didn't know it was possible having slashes before the '?'