PowerShell script to add an activity via OData/REST

I wanted to create some crm activities based on data in a spreadsheet. Looping thorough the csv file would be easy, the challenge came when I wanted to create new activities in Microsoft CRM 2011, specifically appointments. Below is a PowerShell function that I pieced together to create a new activity if you know GUIDs of the contacts for the regarding, required fields. This is a more fleshed out function compared to this

FUNCTION JBM-CRM-CreateActivityODATA {
PARAM([string][parameter(Mandatory=$true)][ValidateSet("Email", "PhoneCall", "Appointment")]$EntityType,
    [string][ValidateSet("crmserver.company.com", "dev-crmserver.company.com")]$ServerName="crmserver.company.com",
    [string][ValidateSet("CRMOrganizationName")]$OrganizationName="CRMOrganizationName",
    [string][parameter(Mandatory=$true)][ValidateScript({ $_ -match("^(\{){0,1}[0-9a-fA-F]{8}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{12}(\}){0,1}$")})]$OwnerGUID,
    [string][ValidateScript({ $_ -match("^(\{){0,1}[0-9a-fA-F]{8}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{12}(\}){0,1}$")})]$RegardingGUID,
    [ValidateScript({-not @( $_ | where {$_ -notmatch("^(\{){0,1}[0-9a-fA-F]{8}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{12}(\}){0,1}$") }).Count})]$ReqiredGUIDs,
    [string]$Subject,[string]$Description,[datetime]$ScheduledStart,[datetime]$ScheduledEnd,[string]$Location
    )

Function CreateActivityParty{
    PARAM($ActivityId,$ParticipationTypeMask,$EntityType,$GUIDs)

    foreach ($Id in $GUIDs){
    $activityParty = @{}
    write-host $Id
    $activityParty.Add("PartyId", @{Id= $Id;LogicalName= "contact"})
    $activityParty.Add("ActivityId", @{Id= $ActivityId;LogicalName= "$($EntityType.ToLower())"})
    $activityParty.Add("ParticipationTypeMask" , @{ Value=$ParticipationTypeMask })
    $json=new-object System.Web.Script.Serialization.JavaScriptSerializer
    $activityPartyData=$json.Serialize($activityParty)
    $activityPartyData

    $url="http://$ServerName/$($OrganizationName)/xrmservices/2011/OrganizationData.svc/ActivityPartySet"
 
    $http_request = New-Object -ComObject Msxml2.XMLHTTP
    $http_request.open('POST', $url, $false)
    $http_request.setRequestHeader("Accept", "application/json")
    $http_request.setRequestHeader("Content-Type", "application/json; charset=utf-8")
    $http_request.send($activityPartyData)
    $http_request.statusText
    $results=$http_request
    }
}

$assembly = [Reflection.Assembly]::LoadWithPartialName("System.Web.Extensions")
$activityObject = @{
ActivityTypeCode=$EntityType
Subject=$Subject
Description=$Description
ScheduledStart=$($ScheduledStart.ToUniversalTime().ToString("yyyy-MM-ddTHH:mm:ssZ"))
ScheduledEnd=$($ScheduledEnd.ToUniversalTime().ToString("yyyy-MM-ddTHH:mm:ssZ"))
Location=$Location
RegardingObjectId=@{
Id= $RegardingGUID
LogicalName= "contact"
}
}

$json = new-object System.Web.Script.Serialization.JavaScriptSerializer
$activityData=$json.Serialize($activityObject)


$url="http://$ServerName/$($OrganizationName)/xrmservices/2011/OrganizationData.svc/$($EntityType)Set"
 
$http_request = New-Object -ComObject Msxml2.XMLHTTP
$http_request.open('POST', $url, $false)
$http_request.setRequestHeader("Accept", "application/json")
$http_request.setRequestHeader("Content-Type", "application/json; charset=utf-8")
$http_request.send($activityData)
$results=$http_request
$ActivityId=$($json.DeserializeObject($results.responseText)).d.ActivityId

# Set Required Attendees
CreateActivityParty -ActivityId $ActivityId -ParticipationTypeMask 5 -EntityType $EntityType -GUID $ReqiredGUIDs
# Set Orgaizer
CreateActivityParty -ActivityId $ActivityId -ParticipationTypeMask 7 -EntityType $EntityType -GUID $OwnerGUID
}