Unfortunately it's not possible to vote on an issue on customer portal interface, although, there is an improvement request open for this matter. Please find it below. We strongly suggest you to vote on this request in order to expedite its resolution.
Until JSD-1743 is resolved, one work-around is to configure the "Introduction text" in Portal Settings and add a hyperlink to a kanban board where users can see the full list of issues and vote on them there. I did this in setting up our idea portal, and I think I like it even better than Service Desk's functionality to let users search for issues created by other people; it actually makes it easier to filter to just the issue type you want them to see. In setting up the kanban board, I just put all statuses in a single column (which prevents drag-n-drop status changing), and I ordered the list by the number of votes.
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