This takes a few steps, but there’s a good tutorial here:
If you want this to run on a separate server, that’s fine – in that case, this isn’t Discourse-specific. You can also host this on the same server, in that case, this tutorial is relevant to get you started:
It’s actually easier to have the two sites in two servers, but you can do it in one. The instructions linked above should help, but If you can’t get the support you need here out get tired of fooling with it I will be glad to help.