Not sure what you mean by "apply some policy to a threat assessment". Threat assessment requests are basically the equivalent of admin submissions, a way to send a message (file/url) for re-examination by Microsoft. Basically, you use it to get a second opinion on why a given item was (not) marked as spam/malware. Nowhere in the process you need to specify or modify the policies mentioned above. Those are not configurable via the Threat assessment API.
I've read about admin submissions feature but I couldn't find any info how to configure admin submissions via Exchange Online PowerShell cmdlets.
If I want to let admin to decide what to do when a user send a report how can I configure via powershell that the report from user won't be send directly to Microsoft but only to a tenant admin in User reported message view?