OK, so you’re not working with a local cloned repo. That explains editing from your master, though I’m still not thrilled about it.
Spawning is a two-step process. First, it needs to be in an item_group (item_groups.json may be of interest here) and that item_group needs to get called in mapgen. So you could add it to existing item groups (hardware groups that contain rebar would be a good fit) and/or make a new one.
However, making new item_groups requires a bit more work, so I’d rather you not mess with that without proper branching and a local compiler.