I'm not sure you can do this by looking at the FullFormattedMessage.
First try to determine which event you actually see for this.
Make sure the action actually occurred during the interval, then from the list of returned events find the one you should be filtering for
$xxxxx= Get-Cluster xxxxx
Get-VIevent -Entity $xxxxx -Start 03/23/2018 -Finish 03/26/2018 -MaxSamples ([int]::MaxValue) |
Group-Object -Property {$_.GetType().Name}
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