Submitting Form to Forms and Mailchimp Apps At Same Time

I have a Mailchimp signup form using your Mailchimp app. How do I submit the form to the Forms app at the same time so I know if someone has subscribed?

Currently the code is:

<perch:form id="subscribe" app="perch_mailchimp" double-optin="true" send-welcome="true" />

<perch:success>
<div class="g-alert type_success with_close">
<div class="g-alert-close"> ✕ </div>
<div class="g-alert-body">
<p>Thank you for signing up, you will shortly receive a welcome email.</p>
</div>
</div>
</perch:success>

<perch:label for="name" class="blocklabel"><perch:content id="firstname" type="text" label="First Name" /></perch:label>
<perch:input type="text" id="firstname" required="true" mailer="FNAME" class="input_bg" />

<perch:label for="name" class="blocklabel"><perch:content id="lastname" type="text" label="Last Name" /></perch:label>
<perch:input type="text" id="lastname" required="true" mailer="LNAME" class="input_bg" />

<perch:label for="email" class="blocklabel"><perch:content id="email" type="text" label="Email Address" /></perch:label>
<perch:input type="email" id="email" required="true" mailer="email" class="input_bg" />

<div class="clearfix"></div>
<div class="submit">
<perch:input type="submit" id="submit" value="Subscribe" class="contact_submit" />
<perch:input type="hidden" value="1" id="confirm" mailer="confirm_subscribe" />
</div>
</div>
</perch:form>