Could you try to see if this command works for you?
$ResourceAccountId = (Get-CsOnlineUser "autoattendant@example.com").ObjectId
$AutoAttendant = (Get-CsAutoAttendant -NameFilter "Main Auto Attendant").Identity
New-CsOnlineApplicationInstanceAssociation -Identities @($ResourceAccountId) -ConfigurationId $AutoAttendant -ConfigurationType AutoAttendant
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