Set Multi-value Lookup Field Value using Powershell and CSOM.ps1

<#
  get $ctx and $site
#>

$web = $site.OpenWeb("news")
$list = $web.Lists.GetByTitle("Posts")
$item = $list.GetItemById(6)
$ctx.Load($web)
$ctx.Load($list)
$ctx.Load($item)
$ctx.ExecuteQuery()

$mvLookup = $item["MultiValueLookupColumnName"]
$mvLookup |% { "Lookup Value: $($_.LookupId):$($_.LookupValue)" }

# create an Object[] and add FieldLookupValue instances
$lookupValueCollection = @()
$lookupValue = New-Object Microsoft.SharePoint.Client.FieldLookupValue
$lookupValue.LookupId = 4
$lookupValueCollection += $lookupValue

$lookupValue = New-Object Microsoft.SharePoint.Client.FieldLookupValue
$lookupValue.LookupId = 5
$lookupValueCollection += $lookupValue

# convert the Object[] to a FieldLookupValue[]
$mvLookup = [Microsoft.SharePoint.Client.FieldLookupValue[]]$lookupValueCollection 

# set multi-value lookup field value
$item["MultiValueLookupColumnName"] = $mvLookup
$item.Update()
$ctx.ExecuteQuery()

Thanks a lot!