How can I set a form to respond to selection (with onchange) with JS instead of HTML

Question

I have this form and I want to prevent it from posting an HTML get request when I select a value.

<%= form_tag study_setup_user_path(@user), id: 'filter-form', method: :get, remote: true do %>

  <%= select_tag(:t, options_from_collection_for_select(@user.tags, 'name', 'name', selected: params[:t]), prompt: 'Select Tag', include_blank: true, class: "form-control", onchange: "this.form.submit()")  %>

<% end %>

In the controller I have the following study_setup action:

def study_setup
  @user = current_user
  @vocabs = @user.vocabs.all
  @tags = @user.tags.all

  if params[:t] != ''
   .
   .
  end

  respond_to do |format|
    format.js
    format.html
  end
end

This should already be enough to make the controller handle it as JS, right? But the controller handles it as HTML. Why?