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Create a JQL that returns the issues you want your customers to vote for (e.g. project = "VOICE" and resolution is EMPTY).
Open your Service Desk project, select Queues => Customers' Voice, insert the JQL, enable it using the toggle and click Save
Go to the portal for this service desk. Here you (and also your customers) can vote for these issues.
The votes are then displayed directly within the issue.
