This PowerShell script updates a custom field on a given Zendesk ticket.

ZendeskAPI-UpdateCustomField.ps1

param(
    [Parameter(Mandatory=$true)][Int32]$ticketID,
    [Parameter(Mandatory=$true)][Int32]$fieldID,
    [Parameter(Mandatory=$true)][string]$fieldValue
)

#Zendesk API Connection Headers Referencing System Environmental Variables for username and API token.
$headers = @{Authorization = 'Basic ' + [Convert]::ToBase64String([Text.Encoding]::ASCII.GetBytes("$($env:ZendeskAPI_Username):$($env:ZendeskAPI_Token)"));}

#Zendesk API: Update Ticket
$uri = "https://yourdomain.zendesk.com/api/v2/tickets/$ticketID.json"

$json = '{
    "ticket": {
        "custom_fields": [
            { "'+$fieldID+'": "'+$fieldValue+'" }
            ]
    }
}'

Invoke-RestMethod -Uri $uri -Method Put -Headers $headers -ContentType "application/json" -Body $json