To update the Last Order Date custom field on the related account when an order is marked complete in Salesforce, the best practice is to use a record-triggered flow:

Create a Record-Triggered Flow:

Use Salesforce Flow to create a record-triggered flow on the Order object.

Set the flow to trigger when a record is updated (specifically, when the order status is updated to complete).

Update the Related Account:

In the flow, use a Get Records element to fetch the related Account record.

Use an Update Records element to update the Last Order Date custom field on the related Account with the date the order was marked complete.

This approach ensures that the data remains within Salesforce and is updated immediately as part of the same transaction, providing a robust and efficient solution.