Change View based on Option selected in select_tag in rails

I am trying to make a form where a user can wither post normally with their username like this - > normal

  =form_for @confession , html: {multipart: true} do |f|
 =f.label :Confess
 =f.text_area :confession , require: true
 =f.file_field :confessionimage
 =f.select (:id,options_for_select(ID))
 =f.submit 'Confess'

or Anonymously where their Names will be hidden and no one will know that who posted this post .. for this what I thought was I will make a user named anonymous in database and if a user select anonymous in select_form while posting they will post as an anonymous user.

for this in my controller I want something like this and main point that I can't understand is how can the controller know what user has selected ?

this is my controller

    def index 
 @amitian = Amitian.where(institute: current_amitian.institute) if                 amitian_signed_in?
 @confessions = Confession.where(amitian_id: @amitian.ids).order('created_at   DESC') if amitian_signed_in?
  @confession = current_amitian.confessions.build 
  @anonymous = Amitian.where(email: anonymous@anonymous.com)
  # (if anonymous selected )
do @anonymous.confessions.build   
 end

can anyone plz help me how can I achieve this ? Plz hhelp m stuck at this part