In Akka HTTP, You can create custom headers by extending ModeledCustomHeader or CustomHeader.

CustomHeaderSpec.scala

package com.tutorial.sample

import akka.http.scaladsl.model.headers.{CustomHeader, ModeledCustomHeader, ModeledCustomHeaderCompanion, RawHeader}
import org.scalatest._

import scala.util.Try

class CustomHeaderSpec extends FlatSpec with Matchers with BeforeAndAfterAll {

  final class ApiTokenModeledHeader(token: String) extends ModeledCustomHeader[ApiTokenModeledHeader] {
    override def renderInRequests = false
    override def renderInResponses = false
    override val companion = ApiTokenModeledHeader
    override def value: String = token
  }
  object ApiTokenModeledHeader extends ModeledCustomHeaderCompanion[ApiTokenModeledHeader] {
    override val name = "apiKey"
    override def parse(value: String) = Try(new ApiTokenModeledHeader(value))
  }

  case class ApiTokenHeader(token: String) extends CustomHeader {
    override def renderInRequests = false
    override def renderInResponses = false
    override def value: String = token
    override val name = "apicustomKey"
  }


  "A custom header" should "be validated" in {
    val ApiTokenModeledHeader(t1) = ApiTokenModeledHeader("token")
    t1 should ===("token")

    val RawHeader(k2, v2) = ApiTokenModeledHeader("token")
    k2 should ===("apiKey")
    v2 should ===("token")

    // will match, header keys are case insensitive
    val ApiTokenModeledHeader(v3) = RawHeader("APIKEY", "token")
    v3 should ===("token")

    intercept[MatchError] {
      // won't match, different header name
      val ApiTokenModeledHeader(v6) = RawHeader("different", "token")
    }

    val ApiTokenHeader(v4) = ApiTokenHeader("token")
    v4 should ===("token")

    // Will not compile
    /*val ApiTokenHeader(v5) = RawHeader("APICUSTOMKEY", "token")
    v5 should ===("token")*/
  }

}

Lines 33-35 don't compile for me.