This is not possible with the JIRA Service Desk Portal.
You could grant browse rights for the project to anyone, but that would make all of your service desk issue available to the entire internet.
You could add all your customers as jira-users and grant them browse rights on the project (that would ofcourse require you to get extra licenses for you customers).
Hello Atlassian Community! I'm Teresa, the Product Marketing Manager for Jira Service Desk Server at Atlassian. I'm excited to announce two exciting releases for Jira Service De...
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