Well, just goes to show that it really is unofficial. The query parameter name should be returnUrl.
https://edk-central.lshtm.ac.uk/-/single/djX34d3EsJGTvdjbvCbIEWz16kZ8oY8?st=XZSPPeWNEw1aX!hpagdyL72QyF87mgX0546BCbVqWdyMSDFPZm9XZENYigH8i7o2&returnUrl=https://google.com
return_url is the name of the query parameter to request such a link. Technically that's really what should be used and will eventually be available from Central at some point (same as building offline links).