I would probably use a partial for this as this doesn't seem abstract
enough to be a helper. Will you use this form differently in more
than one place?
To answer you're exact question, I'm not sure that remote_form_tag is
in FormTagHelper - it's in PrototypeHelper. However, I would
seriously consider using a partial as this helper could get real
messy, real fast.
If you really do need to use a helper, I'd suggest having a look at
the source code for the tag helpers and see how it's handled by Rails.