To activate the Redshift subscription you need to ensure you are using the latest version of the Maxon App from the link below.
https://www.maxon.net/en/downloads/ - You will find a blue download button for the Maxon app at the top of the page
You can then use the Maxon app to install your preferred Redshift Version.
Please note - this is compatible with 3.0.54 and onward only - it is not backwards compatible and cannot be used to license anything before 3.0.54
Then sign in with your MyMaxon account - and Activate Redshift. Licenses are now stored with your account.
There are no activation keys for the Redshift subscription.
If you have a Redshift Node-locked licence, your licensing method has not changed and you still use the Redshift Licensing tool.
This document explains the location of the Redshift Licensing Tool.
Please see this article if you are not seeing Redshift appear in Cinema 4D S26.