Phoenix redirect takes me away from subdomain

I am working on converting an existing application to a multi-tenant one with subdomains enforcing the organization. Tenants have an “admin” panel but can also impersonate their own customers via an impersonator session.

The impersonator session takes a token (decoded with Guardian) adds information about the organization and the impersonator to the session and then redirects to the storefront.
The way the app is setup a redirect from admin to the storefront takes one from localhost:4000 to localhost:4002.

For now I’m just playing with this locally so this isn’t an infrastructure question, per se, but if I redirect from a tenant admin, say customer1.localhost:4000 I loose the subdomain going to localhost:4002. Is there something in the endpoint or a plug I could implement to enforce the subdomain while the impersonation finishes?

For reference:

The AdminApp.ImpersonatonController looks like this (modified for brevity):

 def impersonate(%Conn{} = conn, %{} = params) do

    impersonator = Guardian.Plug.current_resource(conn)
    org_id = Repo.get_org_id()

    claims = %{
      impersonator_id: impersonator.id,
      organization: org_id
    }

    {_, token, _claims} = Guardian.encode_and_sign(impersonator, claims)

    conn
    |> authorize(action: :impersonate)
    |> redirect(external: "#{url}/impersonate?token=#{token}))
  end

Then the StorefrontWeb.SessionController has this action:

 def impersonate(conn, %{"token" => impersonation_token}) do
    conn
    |> ... decode and verify token...
    |> create_impersonator_session()
  end

  defp create_impersonator_session({:ok, token, conn}) do
    organization = token["organization"]

    conn =
      conn
      |> GuardianPlug.sign_in(user)
      |> put_session(:impersonating, {token["impersonator_id"], token["user_id"]})
      |> put_session(:organization, organization)

    redirect(conn, to: successful_sign_in_redirect_path(conn, user))
  end