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First of all you will have to Set up organizational relationships. 1.  Set up organizational relationships. Ø   In the admin portal of Organization A: set up an organizational relationship with domain of Organization B. o    Login to exchange admin center. o    Click on organization. o    Click on sharing. o    Click on + sign below to organization. o    Create a organization relationship there and save the changes. o    Do it again in the other domain. o    In the admin portal of Organization B: set up an organizational relationship with domain of Organization A. Ø   Reference: https://docs.microsoft.com/en-us/archive/blogs/ehlro/enable-freebusy-information-between-the-office-365-users-from-two-different-tenants-when-the-autodiscover-of-one-of-them-is-pointing-on-premise 2.        Run PowerShell so external user able to book the meeting room. Ø   Open Exchange Online PowerShell (EOP) Module and run below command      Connect-EXOPSSession -UserPrincipalName nik.nursyafieqah@enfrasys.co