class ActionView::Helpers::FormBuilder

def rich_textarea(method, options = {})

Please refer to the documentation of the base helper for details.

<% end %>
<%= f.rich_textarea :content %>
<%= form_with model: @message do |f| %>

Wraps ActionView::Helpers::FormHelper#rich_textarea for form builders: 
def rich_textarea(method, options = {})
  @template.rich_textarea(@object_name, method, objectify_options(options))
end