Namespaced Form Objects in Rails

I'm having a hard time trying to get setup some custom form objects in a new Rails 6 project I am building. I suspect this may be due to namespacing issues but I can't yet tell for sure.

app/views/saasy/signups/new.html.erb

<%= form_with(model: [ :saasy, @signup ], url: saasy_signups_path(@signup),  local: true) do |signup_form| %>

  <%= fields_for :account, @signup.account do |account_fields| %>
    Organization name: <%= account_fields.text_field :organization %>
  <% end %>

  <%= signup_form.submit %>
<% end %>

app/controllers/saasy/signups_controller.rb

class Saasy::SignupsController < ApplicationController
  def new
    @signup = Saasy::SignupForm.new
  end

  def create
    @signup = Saasy::SignupForm.new(signup_form_params)
    @signup.register
  end

  private

  def signup_form_params
    params
      .require(:saasy_signup_form)
      .permit(account_attributes: [:organization])
  end
end

config/routes.rb

Rails.application.routes.draw do
  namespace :saasy do
    resources :signups, only: [:new, :create]
  end
end

app/forms/saasy/signup_form.rb

module Saasy
  class SignupForm
    include ActiveModel::Model

    attr_accessor :user, :account

    delegate :attributes=, to: :user, prefix: true
    delegate :attributes=, to: :account, prefix: true

    def initialize(params= {})
      super(params)
      @user = Saasy::User.new(params)
      @account = Saasy::Account.new(params)
    end

    def register
      # eventually do actual signup stuff here
    end

  end
end

However, whenever I test it I get back the following message: param is missing or the value is empty: signup_form

The params hash looks like this:

{
"authenticity_token"=>"BhhvRaYKf220afExocQ//LIY1jszVsXs+lThFeUFKvr6ciVBsa+22mSxwO3yT6mK2uOsWSCKx9gL6WIaGmmvSg==", 
"account"=>{"organization"=>"Example Name"}, 
"commit"=>"Create Signup form"
}

I've tried a whole lot of general messing around solutions like playing with the form_with in the view and changing route names etc but I've not had any luck so far. Any advice would be really appreciated!