Wait a minute ... Is this for logging into the html email address?
If so, you definitely need to rewrite html to tables so that it works with most clients (check in Outlook 07 if you don't believe me).
Also, any form will not work in html-letter because of the blocks designed to protect end users. Please consider changing this as the "Register Now With FooBar" link. and then fill out the form on the web page. In the best case, you will get the minimum speed for completing the form, because users are not used to fill out forms inside the letters themselves.
From the point of view of your original problem, you can try using the float and margin-right css properties to position the button accordingly. Check out jsfiddle .
<input style="float:right; margin-right:12px;" type="submit" value="Subscribe" name="subscribe" id="mc-embedded-subscribe" class="button">
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