Bootstrap's multiple selectpicker using Ruby on Rails

I'm new to Ruby on Rails, and I'm trying to implement Silvio Moreto's bootstrap-select plugin in my web-page.

I already have everything I need installed, but I don't know how to embed my current .erb code into the syntax needed for the selectpicker multiple class to work.

Additionally, I have some other style classes I need to maintain so my page layout remains formatted the way it is.

Here is an example on how the selectpicker should be implemented:

<select class="selectpicker" multiple>
  <option>Mustard</option>
  <option>Ketchup</option>
  <option>Relish</option>
</select>

and this is the part of my code I want to implement it on:

<%= select_tag :portafolio_id,
                  options_from_collection_for_select(current_user.portafolios, :id, :nombre,
                  params[:portafolio_id]), 
                  multiple: true, size: '8%', class: "form-control input-sm "%>