How to use custom domain and HTTPS?

[oncet]

I'm trying to serve a CRA which uses a custom domain and HTTPS through an .env file:

HOST=myapp.localhost
HTTPS=true

When running npm start it opens up in the browser at https://myapp.localhost but when running npx serve -s build it opens up at the standard http://localhost:5000, is there any way to get the same behaviour as with npm start when using serve?

Thank you!