app/helpers/HSTSFilter.scala

package helpers

import org.apache.pekko.stream.Materializer
import play.api.mvc.{Filter, RequestHeader, Result}

import scala.concurrent.{ExecutionContext, Future}

class HSTSFilter()(implicit val mat: Materializer, ec: ExecutionContext) extends Filter {
  def apply(nextFilter: (RequestHeader) => Future[Result])
           (requestHeader: RequestHeader): Future[Result] = {
    nextFilter(requestHeader).map(_.withHeaders("Strict-Transport-Security" -> "max-age=31536000"))
  }
}