No further configuration should be necessary for the tabby language server itself, as I’ve already included a configuration with my Dotfiles.
But in general, you want to add tabby to the language-servers array of a particular language you want to support within the languages.toml file.
Tip
You can either add it to the global file in ~/.config/helix file, or even better configure this on a per-project basis, by using a project-local .helix directory!