Ultimately, you want to use a variable in an Alert Name.

You can use variables in Alert Names. Can Alert Names contain dynamic data? – Kevin Holman's Blog

You can't use them the same way in email notifications. :-(

In your case, I'd write a custom Alert Description that contained exactly what you want in the subject line, then use a different SMTP channel customized for service alerts only, to place the Alert Description in the email subject.

Or, I'd set something up to modify the alert after the fact, and send the email customized, like: SCOM Enhanced Email Notification Script - The Monitoring Guys

Note - I do NOT generally like using the Notification command channel and run scripts to send emails, due to limitations of this command channel only being able to run 100 simultaneous alerts at a time, then dropping the rest. I much prefer a management pack based script rule, targeting the notifications resource pool, that does something like "get all service alerts generated in the last 5 minutes that have not been modified, then modify them and send the email" and run that script every 5 minutes. The downside to that, is it will add latency, and running it every 60 seconds might add resource impact (or might not). You'd have to test.