Surface Hub reservation requests from on-premises accounts don’t get processed

A quick write-up for something I came across today.
When you have a Exchange Hybrid deployment and users with their mailboxes housed on-premises aren’t able to book the Surface Hub then check if the Room Mailbox accepts “external meeting messages” with this PowerShell command from Exchange Online:
Connect-ExchangeOnline
Get-CalendarProcessing SURFACE HUB ACCOUNT | select ProcessExternalMeetingMessages
If the answer is “FALSE” then change this value to TRUE
Get-CalendarProcessing SURFACE HUB ACCOUNT | Set-CalendarProcessing -ProcessExternalMeetingMessages $true
And your issue has been resolved!