As Carl is explaining, you can add a CF all key or a One-touch key, but this will still require interactions on the physical phone. There is not a way to set Call Forwarding from eManager.

You have 2 other options besides the above suggestions.
1) Use the code #6012 to manually set the forwarding from a local phone. This is a remote set of CF-All to another extensions
2) Install the old , retired software MyPhoneManager. It's a bit buggy on current versions of IE, but I think you can set the Compatibly mode.

If the user will be toggling the Call Forwarding periodically (such as when they leave the office), I think assigning a CF-All key is the best solution. Unfortunately you still need to be in front of the phone to set the destination using #9876 user programming mode.