your previous code was correct. To access a folder in other mailbox, use that constructor:
new EwsFolderId(EwsSpecialFolder.Inbox, sSharedMailbox)
You can see in a Debug log, whether the request is targeting your shared mailbox or not. You can create a log as described at https://www.rebex.net/kb/logging/.
Ews.GetMessageList results in FindItem SOAP request
(you can see it under this line:
Invoking 'FindItem' action
and see whether
DistinguishedFolderId element has
Mailbox childe element with correct value or not. Few lines later you can see the response.
If an error occured on the server (like you wrote in your comment), the response looks very similar to this
<h:ServerVersionInfo ... />
<s:Body ... >
<m:FindItemResponse ... >
<m:MessageText>The specified object was not found in the store.</m:MessageText>
If you cannot locate the request and response in the log, please send it back to us at email@example.com for analysis. For now, I can only tell that the user have not sufficient rights to see emails in that other mailbox.