This post function will only work in Company-managed/Classic projects.
Issue Checklist offers a post function to automatically mark checklist items as completed on the indicated workflow transition:
Navigate to Project Settings > Workflows
Click the pencil icon to edit the workflow.
Select Diagram mode and click on the transition where you want to add the post function.
Click on Post Functions.
Click Add post function.
Select the Complete All Checklist Items post function and click Add.
Click Add again to confirm creation of the post function.
Click Publish draft to save the changes.
Test the post function by transitioning an issue with an incomplete checklist(s) to the indicated status. After the transition is made, all checklist items should be marked as completed.
In case there’s more than one checklist on the issue, the items of all the checklists will be checked.