From the plugin configuration the same way you would today.
From a technical perspective, every root server has a unique ID which is a MurmurHash of the root server URL, which I’m calling prefix ID. As long as this doesn’t change the IDs will remain static.
The service body ID is then a combination of that hash + _ + ID.
You can see a list of all the prefixIds https://na-bmlt.org/_/sandwich.
https://github.com/radius314/sandwich/blob/master/server.js#L192