The recommended solution is typically to make your landing page on another server (and use www subdomain). If you want to run both on the same server you can look at Set up Discourse on a server with existing Apache sites. There is a similar topic for nginx. Both assume that you know how to create your html landing page.
Could you detail what would be the purpose of this page, and what it would contain? You mention an HTML page, so it would be static, which can make things easier…
You could have a look at Page Publishing, or make use of CSS to remove unused elements from a page to make it appear as different from your Discourse forum, for example based on this: Page Publishing - #145 by denvergeeks
Though you could find some workarounds like my examples to have a page that differs from your Discourse’s appearance, making it look like it doesn’t belong to Discourse, the JS part would be tricky.
I think the most proper way would be either to have a web server along Discourse on your server. There’s documentation about this:
Or to host your website on a different server, which is much easier. A low-specs, cheap one would perfectly do the job.
You can find such hosting plans at about 2$/month. You can even have a free 100 MB plan on OVH, but I think the domain must be registered on their site.