How to take an action on startup 

A global setting onLoad is of type State => State and is executed once, after all projects are built and loaded. There is a similar hook onUnload for when a project is unloaded.

Project unloading typically occurs as a result of a reload command or a set command. Because the onLoad and onUnload hooks are global, modifying this setting typically involves composing a new function with the previous value. The following example shows the basic structure of defining onLoad.

Suppose you want to run a task named dependencyUpdates on start up. Here’s what you can do:

lazy val dependencyUpdates = taskKey[Unit]("foo")

// This prepends the String you would type into the shell
lazy val startupTransition: State => State = { s: State =>
  "dependencyUpdates" :: s
}

lazy val root = (project in file("."))
  .settings(
    ThisBuild / scalaVersion := "2.12.6",
    ThisBuild / organization := "com.example",
    name := "helloworld",
    dependencyUpdates := { println("hi") },

    // onLoad is scoped to Global because there's only one.
    Global / onLoad := {
      val old = (Global / onLoad).value
      // compose the new transition on top of the existing one
      // in case your plugins are using this hook.
      startupTransition compose old
    }
  )

You can use this technique to switch the startup subproject too.