I think this setting will do what you want:
web.server.protocol=https
This line would go into the portal-ext.properties file. I am fairly certain you will lose it during UMP upgrades, so be ready to add it back in.
I believe I have seen this setting behave two different ways. I am not sure what other settings might affect its behavior, but both seemed to achieve the goal. In one case, all HTTP connections to the UMP sign-in page were redirected to HTTPS. In the other case, the UMP sign-in page was HTTP, but the form submission was HTTPS. This meant that the user credentials were encrypted, and the rest of the session remained encrypted in my testing.