If I understand your answer correctly, what you want to do is explained in the official wiki page here: Create a fake input that does NOT read attributes. You can use a field not related to any real database column by Edward’s suggestion, however you don’t need to define an attribute in your model if the form field is nothing to do with the model.
In summary, the trick explained in the page is defining a custom input called ‘FakeInput’ and use it like this:
<%= simple_form_for @user do |f| %>
<%= f.input :agreement, as: :fake %>
....
Do not forget to restart your rails server after adding/modifying a custom input as Fitter Man commented.
UPDATE: Please note that the official wiki page has updated and the sample code on the wiki page is not working for those which use older versions of SimpleForm. Use code below instead if you encounter an error like undefined method merge_wrapper_options for.... I’m using 3.0.1 and this code works well.
class FakeInput < SimpleForm::Inputs::StringInput
# This method only create a basic input without reading any value from object
def input
template.text_field_tag(attribute_name, input_options.delete(:value), input_html_options)
end
end