live_modal.ex

# in router
scope "/", MyApp do
  ...
  live "/users/new", UserLive.Index, :new
  live "/users/:id/edit", UserLive.Index, :edit
  ...
end

# in live/user_live/index.html.leex
<%= if @live_action in [:new, :edit] do %>
  <%= live_modal @socket, MyApp.UserLive.FormComponent,
    id: @user.id || :new,
    title: @page_title,
    action: @live_action,
    user: @user,
    return_to: Routes.user_index_path(@socket, :index) %>
<% end %>