{%form 'customer' %}
    <div class="email">
        <label for="email">Email</label>
        <input type="email" name="contact[email]" /> 
    </div>
    <input type="text" name="contact[first_name]" /> 
    <input type="text" name="contact[last_name]" /> 
    <input type="text" name="contact[note][DOB]" /> 

    <div>
        <input type="submit" value="sign up" />
    </div>
{%endform%}

so the hack is instead of writing customer we use contact so that without creating a customer registration we take extra data as notes in Shopify.