In the Eclipse client, right click on a project area and select 'Open' to open the project area's editor
In the project area editor, navigate to the Process Configuration tab
Expand Project Configuration > Configuration Data > Work Items > Attribute Customization
Select Default Values and click on the 'Add...' button
Add a new Default Value of Type 'User List'.
In the 'Configuration' section of the User List, add users to the list
Navigate to Project Configuration > Configuration Data > Work Items > Attribute Customization > Types and Attributes page.
In the 'Work Item Types' section, select a work item type.
In the 'Attributes' section, add 'default value' to the 'Subscribed By' attribute
Save change to the project area
For more info you could check the below link
https://www.ibm.com/support/pages/how-automatically-add-users-subscribers-when-creating-new-work-items