Where to put "enablePlugins" in SBT?

Following this tutorial, I am asked to add enablePlugins(WindowsPlugin) to my SBT configuration.

I did this by stating exactly this line in my build.sbt but all I get is "Cannot resolve symbol". Do I need to add the dependency somewhere?

Is this an auto plugin and can anyone explain to me what an auto plugin actually is and how I use it?

UPDATE: My build.sbt looks like that:

name := "ApplicationName"

version := "0.3-SNAPSHOT"

scalaVersion := "2.13.1"

enablePlugins(WindowsPlugin)

mainClass in assembly := Some("application.ConfigEditorApplication")
assemblyJarName in assembly := s"application-$version.jar"

assemblyMergeStrategy in assembly := {
  case PathList("META-INF", xs@_*) => MergeStrategy.discard
  case PathList("reference.conf") => MergeStrategy.concat
  case x => MergeStrategy.first
}

libraryDependencies += "org.apache.commons" % "commons-lang3" % "3.9"
libraryDependencies += "commons-io" % "commons-io" % "2.6"
libraryDependencies += "ch.qos.logback" % "logback-classic" % "1.2.3"
libraryDependencies += "com.typesafe.scala-logging" % "scala-logging_2.13" % "3.9.2"
libraryDependencies += "com.typesafe.akka" %% "akka-actor-typed" % "2.6.3"

libraryDependencies += "org.scalatest" %% "scalatest" % "3.1.1" % "test"
libraryDependencies += "org.scalamock" %% "scalamock" % "4.4.0" % Test
libraryDependencies += "org.mockito" % "mockito-scala_2.13" % "1.11.3"
libraryDependencies += "org.mockito" % "mockito-scala-scalatest_2.13" % "1.11.3"