Add the following line to your plugins.sbt file:

addSbtPlugin("com.alejandrohdezma" % "sbt-scalafix-defaults" % "0.16.0")

You'll also need to provide the sbt-scalafix plugin.

This plugin is cross-published for sbt 1.x (Scala 2.12) and sbt 2.x (Scala 3); the line above resolves the right artifact automatically.

The included plugin activates automatically. It will add some default dependencies to scalafixDependencies, enable scalafixOnCompile(except when running on CI) & semanticdb by default and create a .scalafix.conf in your project's root folder with this content.

You can add the .scalafix.conf file to the repository's .gitignore, since it's going to be automatically re-created on every build.

Extra configurations can be added to a file named .scalafix-extra.conf at the root of your repository. The content of this file will be automatically appended to the auto-generated .scalafix.conf.