Allow External senders to create meetings to a room mailbox.

This one is easy. Well hard to find, but easy.

When connecting to Exchange with powershell (Onprem or Office365)

get-calendarprocessing mailboxname | fl

You will get the following listing:

And ProcessExternalMeetingMessages with be set to False

Just type set-calendarprocessing mailboxname -processexternalmeetingmessage $true

and You are done.