Unsubscribe Page - Adding Additional Birthday Field

Hi, 

I am in the process of customising my email preferences/unsubscribe page using the Klaviyo template code as a starting point.

I am wanting to add an additional field (same as the Email address/first name/last name fields) where customers can record their birthday. The code for these fields looks like this…

</div>
            </div>
            <div class="form-group">
               <label for="last_name" class="col-sm-3 control-label">Last Name</label>
               <div class="col-sm-9">
                  <input type="text" class="form-control" id="last_name" name="$last_name" value="{% if request.POST|lookup:'$email' %}{{ request.POST|lookup:'$last_name' }}{% else %}{{ person.last_name|default:'' }}{% endif %}" />
               </div>
            </div>

What elements in the above code (the Last Name field)  would I need to alter in order to have it correctly capture birthday data. 

Thanks

Scott