If you are looking for something similar to npm install --save, then this does not exist in Elixir. You install things by adding them to deps: in the mix.exs file in your project then running mix deps.get.
The other way you may wish to install certain applications is via a mix archive allowing this mix task to be run globally. One example of this is phoenix.new for creating new phoenix applications which can be installed by running:
mix archive.install https://github.com/phoenixframework/phoenix/releases/download/v0.16.1/phoenix_new-0.16.1.ez