This page allows you to generate a game by uploading a yaml file or a zip file containing yaml files. If you do not have a config (yaml) file yet, you may create one on the Player Settings page. If you would like more advanced options, the Weighted Settings page might be what you're looking for.
This game will be generated in race mode, meaning the spoiler log will be unavailable, roms will be encrypted, and single-player games will have no multidata files.
After generation is complete, you will have the option to download a patch file. This patch file can be opened with the client, which can be used to to create a rom file. In-browser patching is planned for the future.