Escape URI String with PowerShell
$EscapedBusinessGroupName = [URI]::EscapeDataString($BusinessGroupName)
$URI = "/identity/api/tenants/$($TenantId)/subtenants?`$filter=name%20eq%20'$($EscapedBusinessGroupName)'"
$Response = Invoke-vRARestMethod -Method GET -URI $URI -Verbose:$VerbosePreference