How to conditionally pass a parameter

pass me maybe

<%= form_tag catalog_index_path, :method => :get, :id => "search-form-header" do %>
  <%= search_as_hidden_fields(:omit_keys => [:q, :search_field, :qt, :page]).html_safe %>    
  <%= text_field_tag :q, params[:q] unless params[:q] == 'dashboard'?, :class => "q", :id => "search-field-header", :placeholder => "Type keywords in here"  %>  
  <%= params[:controller] %>
  <%= submit_tag 'Go', :id=>'search-submit-header' %>
<% end %>