You can enable HTTPS decryption using Tools > Fiddler Options > HTTPS. Without repro steps, I'm not sure I understand what you are referring to when you say "Exchange PowerShell sessions"-- if you elaborate, I may be able to give you more specific instructions.
Generally speaking, however, it's important to understand that Fiddler is a HTTP/HTTPS debugger, and Exchange does not typically use the HTTPS protocol. Even when configured to use RPC-over-HTTPS, the underlying protocol isn't really HTTP, it is merely bootstrapped over a TLS connection that masquerades as HTTPS to circumvent firewalls.
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