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For this you need two projects. One that contains the issues your customers should vote for (must be a software or business project) and the second one has to be a service desk project.

Please note:
In order for your customers to see what they are voting for, these issues should be visible to them.
(see: https://support.atlassian.com/jira-cloud-administration/docs/allow-anonymous-access-to-projects/ or https://support.atlassian.com/jira-service-management-cloud/docs/give-jira-users-permission-to-view-service-project-issues/)