Rails: form_for in application wide footer - ruby-on-rails-5

Question

I started learning Rails for about a month now and I am working on a fairly simple project to improve my skills. It's a blog where editors can add articles and users can subscribe to a newsletter by adding their email. The homepage is an index view and their is a footer that shows up across all pages.
Here is my problem: I would like to include the form_for the newsletter on the footer that exists inside the application layout. That form has a specific model: Subscriber, which stores users emails.
What I've done so far is include the following on the Articles controller:
def index
#articles = Article.order(created_at: :desc).all
#subscriber = Subscriber.new
end
def create
#subscriber = Subscriber.new(subscriber_params)
if #subscriber.save
redirect_to '/home'
else
render 'new'
end
end
def subscriber_params
params.require(:subscriber).permit(:email)
end
In application.html.erb :
<%= form_for(#subscriber) do |f| %>
<%= f.email_field :email, :placeholder => "email address" %>
<%= f.submit 'Sign up', :id => "signup" %>
<% end %>
The form is displayed correctly on the index page only and it doesn't save to the database (without error).
I have tried using a before filter on the application controller as well as rendering a partial without any success.
Edit
Subscribers controller code:
def new
#subscriber = Subscriber.new
end
def create
#subscriber = Subscriber.new(subscriber_params)
if #subscriber.save
redirect_to '/home'
else
render 'new'
end
end
private
def subscriber_params
params.require(:subscriber).permit(:email)
end
Application controller:
before_action :create_new_subscriber
def create_new_subscriber
#subscriber = Subscriber.new
end