Configure issue key in "Create a new issue or add a comment to an existing issue" mail handler?

[Clara Tatem]

I'm using this option but it doesn't create the task with an issue key so customers can use it in their mails  to add comments to their requests (existing issues).

How do I do this?

Custom email handlers

Create a new issue or add a comment to an existing issue

This handler creates a new issue or adds a comment to an existing issue. If the subject contains an issue key, the message is added as a comment to that issue. If no issue key is found, a new issue is created in the default project.

[Nic Brough -Adaptavist-]

All tasks are created with a key, it's one of the few pieces of completely mandatory data, so I'm a little confused as to what the problem here is?