concatenate @ to string in rails form

I have a rails form that takes in a slack username. In my view it concatenates a "@" to it as follows:

<div class="col-md-5ths col-xs-6"><h3>Slack </h3><h2> <%= "@#{@user.slack}"%></h2></div>

How would I have the edit form append the "@" so the user know's it's there?

Current form

<%= f.label :slack, "Slack username" %>
<%= f.text_field :slack, :maxlength => 30, class: 'form-control' %>