getting value of collection_select

<%= collection_select(:post, :id, @posts, :id, :title, options ={:prompt
=> "-Select categories-"}, :class=>"category_dropdown_list") %>

Hi, how would I send the value of the selected item in the
collection_select?

I'm trying to use an observe_field for an input, and in the action I'll
perform the steps corresponding to the collection_select.

Thanks