I was able to do so in the GUI as I mentioned before.
@Falco, @pfaffman, @Jagster, @merefield… thanks to you all, I have successfully setup the reverse proxy and I no longer have those Mixed Content errors.
Once I reverse proxied into port 80 of the discourse VM and I was able to register and such, it came down to setting force_https using the GUI and adding the x-forwarded-proto flag in the advanced tab of nginx proxy manager.