Backend workflow triggering as a user?

Hey there, I’m trying to get a backend workflow to trigger a workflow (posting to a Slack channel as a user), but is there a way to receive a webhook (ie with an email address, say) that then parses that request and posts to Slack as that user? I don’t think there is, but thought I’d check.

When I try with just a “Post to slack” command, it says “user is not logged in” which makes sense.