How can I append a text_field_tag to a form_for?

You probably just have a typo in your code...

If you include a

   <%= text_field_tag 'firstname' %>

in your form, then you can get the value in the controller using

   params[:firstname]

If you use nested resources, a

   <%= text_field_tag 'user[firstname]' %>

in your form, will result in

   params[:user][:firstname]

in your controller.

Regards,
Tom