session_controller.ex

defmodule MangoWeb.Admin.SessionController do
  use MangoWeb, :controller
  plug :set_layout

  alias Mango.Administration

  def new(conn, _params) do
    conn
    |> render("new.html")
  end

  def send_link(conn, %{"session" => %{"email" => email}}) do
    user = Administration.get_admin_by_email(email)
    conn = case user do
      nil ->
       conn
       |> put_flash(:error, "Authentication error")
      user ->
       link = generate_login_link(conn, user)
       conn
       |> put_flash(:info, "You magic login link is #{link}")
      end
     conn |> render("new.html")
   end

  defp generate_login_link(conn, user) do
    token = Phoenix.Token.sign(MangoWeb.Endpoint, "user", user.id)
    admin_session_url(conn, :create, %{token: token})
  end

  defp set_layout(conn, _) do
    conn
    |> put_layout("admin_login.html")
  end
end

Where is the "admin_session_url" on line 28 coming from?

Nevermind... this is only an issue when working with Phoenix 1.4. For anyone else that ran into this problem, you'll need to use "Routes.admin_session_url(conn, :create, %{token: token})"