neovim-flake/docs/manual/custom-plugins/configuring.md

Configuring

Just making the plugin to your neovim configuration available might not always be enough. In that case, you can write custom vimscript or lua config, using config.vim.configRC or config.vim.luaConfigRC respectively. These options are attribute sets, and you need to give the configuration you're adding some name, like this:

{
  # this will create an "aquarium" section in your init.vim with the contents of your custom config
  # which will be *appended* to the rest of your configuration, inside your init.vim
  config.vim.configRC.aquarium = "colorscheme aquiarum";
}

:::{.note} If your configuration needs to be put in a specific place in the config, you can use functions from inputs.neovim-flake.lib.nvim.dag to order it. Refer to https://github.com/nix-community/home-manager/blob/master/modules/lib/dag.nix to find out more about the DAG system. :::

Also, if you successfully made your plugin work, please make a PR to add it to the flake, or open an issue with your findings so that we can make it available for everyone easily.