Run Powershell as administrator
Install-Module -Name ExchangeOnlineManagement -RequiredVersion 2.0.3
You will need to respond Yes to 2 prompts
$UserCredential = Get-Credential
make sure the credentials you enter have authority to admin the domain
Connect-ExchangeOnline -Credential $UserCredential -ShowProgress $true
Set-OrganizationConfig -AllowPlusAddressInRecipients $true
May take up to an hour to be fully enabled.