I found it, not sure why it's not auto-populated for us, it's the Vault server URL. You should change the label of "Server URL" to "Vault Server URL", we're configuring a MAIL server here, it's not very intuitive!
The URL field is used to generate emails. Because some sites use a different internally accessible name and externally accessible name for their Vault server, the URL field allows them to enter the URL that will be useful to all users. If we autogenerated it, it could likely be wrong.