+1 to Pete's assertion that Schedules are for exactly that, running processes at a scheduled time or a scheduled frequency.
If what you're after is an email hot-trigger, there are a few solutions, but each of them has their drawbacks.
1) If you have an additional license, you can set up a bot to run as a process administrator, checking for emails and hot folders and triggering the associated bots as appropriate. The downside is that this consumes a license to do it, so only really makes sense if you have multiple processes that require hot-folder or email triggering
2) Set your process on a frequent repeating schedule to check every 5 minutes (or whatever interval you wish). Usually the process doesn't HAVE to be started the second the email arrives and in this case a short delay is often acceptable.
3) Utilise a script to do the checking of the emails and then trigger the process using AutomateC (the Blue Prism command line application)
There are more possibilities than that, but those are the 3 that spring immediately to mind. Of course if the requirement is actually just to allow a user to trigger the process ad-hoc, then email may not need to be part of the solution. In that case you could look at using a script of small custom app to trigger the process directly via AutomateC.
EDIT: After re-reading your initial post, it seems like using AutomateC directly could come the closest to what you're trying to do as it allows you to send a command to Blue Prism to start a process.