form-phx-value.ex

<%= f = form_for :select_user_type, "#", [phx_change: :select_user_type, phx_value_id: creds.id, class: "ui form"] %>
  <%= select(f, :role, ["Admin": "admin", "User": "user"]) %>
</form>

def handle_event("select_user_type", %{"select_user_type" => %{"role" => role}}, socket) do
  IO.inspect socket.assigns
  {:noreply, socket}
end

<form action="#" class="ui form" method="post" phx-change="select_user_type" phx-value-id="1">
...