API v3 Add Attachment To Ticket

This is the code that I have cobbled together based on various resources, such as the public beta documentation and the API documentation in my tenant.

$ApiKey = "API-KEY-GOES-HERE" # Technician API Key
$SdpUri = "https://servicedesk.contoso.com"
$RequestID = "111111"
$input = @"
{
"attachment": {
"filename": "@C:\temp\test.txt",
"addtoattachment": true,
}
}
"@
$header = @{TECHNICIAN_KEY=$ApiKey}
$params = @{input_data=$input;format='json'}
$Uri = $SdpUri + "/api/v3/$RequestID/attachment"
$result = Invoke-RestMethod -Method GET -Uri $Uri -Headers $header -Body $params -ContentType "application/x-www-form-urlencoded"
$result

Could someone please point me in the right direction as to formatting my JSON input? I have a separate script that I am able to get attachments with but I would like to be able to add an attachment to an existing ticket. Currently I am receiving a status_code 4007 "failed" when attempting to run the above code.