Hello Taylor, 

You can do this in a couple of ways. 

1. Using liquid in the template. For example, in the signup page template you will find the liquid tag for this input:

<td><label for="signup_email">Email</label><br>{% text_field "email", class:"text" %}</td>

Here you can add a value to the tag using: value:"XX"

{% text_field "email", class:"text", value:"XX" %}

Which will in turn render:

<input class="text" id="signup_email" name="signup[email]" type="text" value="XX">

2. Using javascript in the template. For example, pasting the following will also change the default input value in the field:

<script>
$(document).ready(function() {
$("#signup_email").DefaultValue("XX");
});
</script>

Which will in turn render:

<input class="text" id="signup_email" name="signup[email]" type="text" value="XX">