HelpDesk uses a powerful JIRA user to provide users interfaces and do some system actions.
You must define it before using the plugin. It must have powerful rights in JIRA. Especially, it must have next rights: system admin, search users, browse project, create issues, create comments and attachments.
You have to specify the user in JIRA Administration menu/Add-ons/HelpDesk:
HelpDesk Super User provides all issues operations on behalf of other users in HelpDesk.
Notice that this Super User requires 1 JIRA license (this can be already existing JIRA-administrator).