With your information, related posts and PowerShell documentation I could solve my problem, but it's not trivial.
I had to use PowerShell commands :
$LiveCred = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri ps.outlook.com/powershell -Credential $LiveCred -Authentication Basic -AllowRedirection
Import-PSSession $Session
Add-MailboxFolderPermission -Identity yyyyy@xxxxx.onmicrosoft.com:\Calendrier -User zzzzz@xxxxx.onmicrosoft.com -AccessRights Editor
==> Remark : Calendrier is french name for english Calendar name folder
Remove-PSSession $Session
Thanks Cherry, this post is closed!