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