Why? The link now goes to FMC URL which require creds to login. User requires an active reservation to obtain creds. Therefore linking to the sandbox URL.
I think you want to point to the Sandbox URL that the user needs to reserve in order to get the credentials. Here's what the email looks like, maybe add this screenshot to help explain this (odd) reservation type?
Learning labs:
Section: Under "How to Setup your own computer" - edit "FMC REST API Sandbox" link to point to - https://devnetsandbox.cisco.com/RM/Diagram/Index/1228cb22-b2ba-48d3-a70a-86a53f4eecc0?diagramType=Topology
Why? The link now goes to FMC URL which require creds to login. User requires an active reservation to obtain creds. Therefore linking to the sandbox URL.