Thank you for your post.
The Get (AccountName) function returns the name of the account. If the AccountName is different than the initials in the Account Manager field, then you may want to invoke a startup script (under File -> File Options...) that includes:
If [ Get ( AccountName ) = <the account name in questions> ]
Set Field [ <a global storage field> ; <the initials of the Account Manager> ]
A global storage field is a field that contains the same result for every record. Each user who opens a shared file has their own value in the global field.
Now, you can set your Access Privileges to:
Account Manager = <global storage field>
... and only those users that sign in as the Account Manager will view those records.
Please let me know if you need clarification for any of the above steps.