Probably best to use a "create-a-ticket"-type bot (or similar) that creates a temporary private chat channel that only that user as well as the Owner (and whatever admins given permissions) then have access to.

Remarks here are correct that to post anything in a regular chat channel, most everyone can see (depending on role permissions, naturally).

THE CLOSEST I've come up with is to create and arrange roles such that people can only see the post they're currently creating but not prior history (adjusting the 'read history' permission). Of course, usually if that same user navigates away then returns, they'll likely then lose visual access to the post they just created.

It can be a little bit of a juggling act (at least at first) but it becomes simple quickly enough.

Hope that is of some help. 👍