You may know that Issue Forms for Confluence does not support the customer portal configuration of the project type Service Desk.
You can use the Issue Form - create form, if you follow the Step-by-step guide.
Change the Permission Scheme (must be done by a Jira Administrator).
Grant the permission Create Issue to Application role - Any logged in User.
Grant the permission Browse Projects to Reporter.
Change the Screens (only required for version < 2.1.0 and must be done by a Jira Administrator).
Remove the Field Request participants from the issue create screens of the project.
Now you can use the Issue Forms - create form. The form will be shown to all users who have the permission to login to Jira.