My main goal with this is to be able to add a new request (which I've already tested successfully) and then add a reply to it. So far I'm having trouble with the reply part. In digging through the API v3, it doesn't appear that there is a reply function (and I don't think that Update Request is the appropriate one unless I'm mistaken) built into v3, so I think I'll have to use V1. However I'm not quite sure how to. 

Here's the code I have, which I think is a mash of V1 and V3:

$ApiKey = "My key goes here"

$SdpUri = "https://servicedesk.mydomain.com"

$RequestID = "111111"

$input = @"

{

    "operation": {

        "details": {

            "to": "name@email.com",

            "subject": "Reply Test",

            "description": "This is a reply test from Powershell"

        }

    }

}

"@

$header = @{TECHNICIAN_KEY=$ApiKey;OPERATION_NAME='REPLY_REQUEST'} 

$params = @{input_data=$input;format='json';}

#$Uri = $SdpUri + "/api/v3/requests"

$Uri = $SdpUri + "/sdpapi/request/$RequestID"

$result = Invoke-RestMethod -Method POST -Uri $Uri -Headers $header -Body $params -ContentType "application/x-www-form-urlencoded"

$result

If someone could point me in the right direction, I would greatly appreciate it.