Session cookie with Phoenix LiveView

You can use phx-trigger-action to make sure a form submit goes through http instead through the socket.

On the example below, when @trigger_submit is true then the form will be sumitted to the path Routes.user_session_path(@socket, :create), which will hit the controller and set the cookie:

<%= f = form_for @changeset, Routes.user_session_path(@socket, :create), phx_trigger_action: @trigger_submit, phx_change: :validate, phx_submit: :submit, as: :user %>
  <%= input f, :email %>
  <%= input f, :password %>
  <%= submit "Sign In", disabled: !@changeset.valid? %>
</form>