Combining GetResponse "Submit" and "Next" buttons

Question

I have a three step sign up form to get people on my GetResponse list.

These three tabs are:

1. The first tab has a question on how you heard about us, with a "Next" button to take you to the second tab.
2. The second tab has my GetResponse Form where the user will enter their email address and click on the GetResponse "submit" button to be added to my list. They will then click on the "Next" button to be taken to...
3. The third tab which has an offer.

Here's the code for my GetResponse Form:

<script type="text/javascript" src="https://app.getresponse.com/view_webform_v2.js?u=cghU&webforms_id=3519704"></script>

Here's the code for the "Next" button which takes you to the next tab:

<tr><td><button class="btn" type="button" onclick="return toggleStep(2)"><span >Next</span></button></td></tr>

What I would like to do is combine this "Submit" button with the "Next" button. In other words, the user provides their email address and clicks on only the "Next" button (without the need to click on the "Submit" button of the GetResponse form). Clicking on this "Next" button will automatically add these people to my GetResponse list and take them to the third tab.

I've tried a number of JavaScript and other "tricks" but nothing seems to work. Is this possible? If so, could you provide me with the code?

Thanks