Yes, I create a policy into SafeSquid Proxy such a way that all the users will be blocked on LinkedIn except User:
umashankar.gutti@gmail.com can only login to LinkedIn.
Create a Profile to Identify LinkedIn Login underRestriction Policies : Access Policies as Show below:
Select Linkedin Login from Request-Types Magic Suggest List
Request-Types: “Linkedin Login” -> This will identify the Application
Give it a Name [PERMITTED LINKEDIN LOGINS]
Policy 1

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Create Policies under Content Modifiers >> Rewriting Policies
In first policy we forbid all users to login into LinkedIn.
For each allowed Email ID create a Rewriting Policy to re-modify the POST data.
Session_key is a field which carries users Email ID replace with SafeSquid_session_key.
Policy 2 :
In second policy we will allow user: umashankar to login to LinkedIn.
Policy 3 :

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