Ruby on Rails Simple_Form Textfield with javascript - ruby-on-rails

Question

I have an application with a large Text field:
<%= simple_form_for(#language) do |f| %>
<div class="form-inputs">
<%= f.input :text_message, :input_html => { :class => "span8", rows: "12" } %>
</div>
And I'm trying to add this HTML and Javascript which allows me to click a button to add a predetermined value into the textfield. Right now it works but I can't figure out how to link it to the RoR textfield.
<form name="virtual">
<input type="text" name="text"/>
<input type="button" onclick="last_name()" value="Last Name" style="border:none;"/>
<input type="button" onclick="first_name()" value="First Name" style="border:none;"/>
</form>
<script type="text/javascript">
function last_name(){
document.forms["virtual"]["text"].value += "{last_name}";
}
function first_name(){
document.forms["virtual"]["text"].value += "{first_name}";
}
</script>
So my question is, how can I combine these two so that selecting the buttons will add "{last name}" or {first name} to the text_message field in the Language model, and not the plain html form.
Edit: To provide more context - later on these variables like {last_name} will be parsed out and replaced with their associated fields - but for now that's not relevant as all I'm trying to do is create buttons that input the actual string "{last_name}" or "{first_name}" into the textfield. This JS was the only way I could figure out to do this
Thanks!