Is there an easy way to automatically capture the sign up date in an opt-in form without requiring input from the subscriber?

When I add auto responders that don't have the custom date field, I get this screen:

When I add the custom field of "Date", I get this screen with additional options:

So, the question is, how do I edit the opt-in form so that the Date field is hidden (doesn't require subscriber input) and auto populates in Sendy based on the sign up date?

<form action="http://mydomain.com/subscribe" method="POST" accept-charset="utf-8">
    <label for="name">Name</label><br/>
    <input type="text" name="name" id="name"/>
    <br/>
    <label for="email">Email</label><br/>
    <input type="text" name="email" id="email"/>
    <br/>
    <label for="Date">Date</label><br/>
    <input type="text" name="Date" id="Date"/>
    <br/>
    <input type="hidden" name="list" value="c"/>
    <input type="submit" name="submit" id="submit"/>
</form>