Get-OwaVirtualDirectory | fl name,externalurl

Name : owa (Default Web Site)
ExternalUrl : https://mail.1ask2.com/owa

EX1 has the mailbox role.
Ex2 has the Client Access Server role.

Set-ExchangeServer -id EX1 -InternetWebProxy 'http://EX2.1ask2.com'

Set-OwaVirtualDirectory -id 'EX2\owa (default web site)' -CalendarPublishingEnabled $true

Set-SharingPolicy -id 'Default Sharing Policy' -Domains 'Anonymous:CalendarSharingFreeBusyDetail'

Select "Publish This Calendar to Internet"

Send the link to a Internet user.