With Webex app installation, there is a switch to auto-populate email for auto sign on, BUT it only supports getting it from SAMAccountName and userPrincipalName from Active Directory, which doesn't offer much help as you need email address to sign in.
msiexec /i Webex.msi EMAIL=$SAMAccountName ALLUSERS=1
msiexec /i Webex.msi EMAIL=$userPrincipalName ALLUSERS=1
There should be support to get mail field from AD as well to make it work properly with auto sign in.
msiexec /i Webex.msi EMAIL=$mail ALLUSERS=1