According to the topic and response below, you cannot force HTTPS server-side on custom domains on Glitch:
As said in the other threads, it’s not possible to force it globally for all projects because it may result in some problems. Therefore, I suggest to make an option on Glitch to force HTTPS on custom domains.
This is a feature suggestion so don’t forget to vote for it if you think it would be useful.
Thanks for the suggestion! I just did that. However, I think it would be cool to have it as an option without leaving Glitch. This option is available on fly.io which is used by Glitch to serve custom domains, so I believe it shouldn’t be too hard to implement.