Can I pre-populate the Order ID field for Shopify in Zendesk?
No, the Shopify Order ID needs to manually be entered in Zendesk. The Order ID is used by the Zendesk app to look up details of the Shopify order and render them in the app. When a customer submits a ticket, the support agent needs to work out what Shopify order it is related to and manually enter the Order ID in the field.
For more information, see the article: Setting up the Zendesk Support for Shopify integration.