Even though the docs say 12, I think this approach should still work for you in v11.
https://docs.xperience.io/k12sp/developing-websites/starting-with-mvc-development/adding-an-administration-redirect-to-mvc-sites
If they hit the raw domain on your admin site itself, you can use a rewrite rule in your admin tool's web.config file.
<configuration>
..
<system.webServer>
<rewrite>
<rules>
<rule name="AdminRedirect" stopProcessing="true">
<match url="^$" />
<action type="Redirect" url="/admin" />
</rule>
</rules>
</rewrite>
</system.webServer>
..
</configuration>