I assume "assign to engineer" is a status in your workflow.
The transition can be setup in the service desk automation settings.
Full explanation can be found here:
I do not think post function will work because the post function will only be activated when the workflow transition happen. You need a trigger event to make the transition. From what I understand you want to make the status transition when a comment is added.
There is a template available to auto transition when comment is added - see attached screen. (service desk server 3.8.1, should be the same on cloud)
Hello Atlassian Community! I'm Teresa, the Product Marketing Manager for Jira Service Desk Server at Atlassian. I'm excited to announce two exciting releases for Jira Service De...
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
You're one step closer to meeting fellow Atlassian users at your local meet up. Learn more about AUGs