Open PowerShell with Admin:
Import-Module msonline
Connect-MsolService
$UserCredential = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic –AllowRedirection
set-executionpolicy unrestricted
Import-PSSession $Session
$mailbox=get-mailbox -resultsize unlimited
$mailbox | ForEach-Object {Add-MailboxFolderPermission $_":\Calendar" -User "[email protected]" -AccessRights Owner}