Sub-task Assign Plugin

Sub-task assign plugin allows you to automatically set the assignee of the sub-task to the reporter/assignee/project lead of the parent task.

To know more about workflow post-functions in JIRA click here.

Installing the plugin

The Sub-task assign plugin is available through Atlassian Marketplace.

Please refer to the documentation available here for instructions on how to install a plugin in JIRA.

Login to JIRA as administrator. Go to the administration screen. and click on Issues and select Workflows.
Choose to edit the workflow in which you would like to add the post-function.
In this example, we will set it up such that the sub-task is automatically assigned to the parent issue task assignee upon creation.Click on the first step. In the screen shot below, I am choosing "Open" step.
Select the Create transition.
Click on Post-function and click Add.
You will have a new option here that is provided by the Sub-task assign plugin. Select Sub-task Assign and click Add.
Select who you would like to assign the sub-task to and click Add.
Save and publish your workflow. Test your workflow by creating a parent issue and then a sub-task to that issue check if the assigee of the parent is being assigned to the sub-task.