Ruby on Rails

I have two models each with their own resources: Organizations and Members. An organization has multiple members and a member is only a member to one organization.

How can I create a simple nested form that signs up a new organization as well as one member for that organization?

My current attempt is below. On submitting the form, this however generates the error message

Organization model

has_many :members, dependent: :destroy
accepts_nested_attributes_for :members, :reject_if => :all_blank, :allow_destroy => true

Member model

belongs_to :organization

Organizations controller

def new
  @organization = Organization.new
  @member = @organization.members.build
end

def create
  @organization = Organization.new(organizationnew_params)
  @member = @organization.members.build(organizationnew_params)
  if @organization.save && @member.save
    @member.send_activation_email  #Will Rails be able to get to this method? The send_activation_email method exists in multiple models and Rails should here use the version from the Member model.
    flash[:success] = "Please check your email to activate your account."
    redirect_to root_url
  else
    render 'new'
  end
end

def organizationnew_params
  params.require(:organization).permit(:org_name,
                                       :phone,
                   member_attributes: [:email,
                                       :username,
                                       :admin,
                                       :password,
                                       :password_confirmation
                                       ])
end

Organizations new view

<%= form_for(@organization) do |f| %> 
  <%= f.text_field :org_name, %>
  <%= f.text_field :phone %>
 
  <%= f.fields_for :member do |p| %>
    <%= p.text_field :username %>
    <%= p.email_field :email %>
    <%= p.password_field :password %>
    <%= p.password_field :password_confirmation %>
    <%= hidden_field_tag :admin, true %>
  <% end %>

  <%= f.submit "Sign up", class: "formbutton btn btn-default" %>
<% end %>

What am I doing wrong? How can I create a simple nested form?