In my Repo module, I have the following:

defmodule Hello.Repo do
  use AshPostgres.Repo,
    otp_app: :hello

  # Installs Postgres extensions that ash commonly uses
  def installed_extensions do
    ["ash-functions"]
  end
end

Are you using Ash 3 or 2? I think this got added recently in Ash 3. Official instructions are here: Get Started With Postgres — ash_postgres v2.10.0

There isn’t much information in your post so I’m assuming you have setup the AshJsonAPI as described here: Getting started with AshJsonApi — ash_json_api v1.7.0 . With that assumption, I’m guessing this is what you need on the User resource to get you the API you need.

defmodule Hello.User do
  use Ash.Resource,
    data_layer: AshPostgres.DataLayer,
    domain: Hello.Users,
    extensions: [AshJsonApi.Resource]

  postgres do
    table "users"
    repo Hello.Repo
  end

  attributes do
    uuid_primary_key :id

    attribute :first_name, :string, public?: true
    attribute :date_of_birth, :date, public?: true
    .... other attributes
  end

  json_api do
    type "user"

    routes do
      base "/users"

      get :get_user, route: "/find"
    end
  end

  actions do
    read :get_user do
      argument :first_name, :string do
        allow_nil? false
      end

      argument :date_of_birth, :date do
        allow_nil? false
      end

      filter expr(first_name == ^arg(:first_name) && date_of_birth == ^arg(:date_of_birth))
    end
  end

end

Basically, you are missing the filter macro in your read action. In the get route, the first argument :get_user is the read action name defined inside actions and the route option is given to make url as GET /users/find, otherwise the default route for a get path will result in /users/:id where :id is a placeholder and is not relevant in your case.

The API will return all the fields marked as public?: true in the attributes section.