How to install Flarum with Composer?

I am trying composer on some projects and wondering... actually I have installed a CodeIgniter 4 by composer in my httpdocs/ folder. Now I would like to install Flarum (forum) as well to have it referenced in my project (like domain.tld/forum links to flarum instance). Questions is: How do I get both up and running koows in the same space? I don't want to use a subdomain for Flarum.