Hello James,
As far as I know, it is not possible to configure a softkey to log in / log out of the extension mobility.
First thing, make sure you created user phones also. You will have the log on those phones with their personnalized profiles.
To have them log in (and log out) on those phones, they will have to press the Settings button (the open book icon on the phone).
Then, go to CME Services (option 4 I think on 504 phones).
And then, ExtensionMobility (option 3)
They will have to enter their username password, press the Submit button and they will be logged in.
You have to do the opposite when they want to log out. (option 4, then option 3, then Logout? )
Remember, a user can only be logged on one phone at a time, so you will have to create as many user profiles as you will have employees at the hot desk at the same time.
Also, I prefer to create usernames and password that are easy to enter, such as adgj , dgjm etc
You will see why when you get to the login menu.
Regards!