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.
Steve