(JIRA OnDemand - Using Chrome and MAC OSX)

Located a user that is assigned an issue, then went to Notification Helper. In the dialog box I enter:

Username
Issue Key
Notification Event - Issue Assigned

Result - The user and Issue Key fields do not autofill.

Workaround:

Open another browser tab and locate the same user assigned to the issue
Open the issue
Copy the issue key to clipboard
Switch back to the Notification help in the previous tab
Start typing in the issue

The issue field will immediately autofill with said issue.