How can I attach an email capture to a typeform?

You can pass the email you captured outside of the typeform via hiden field when you set it up for your form.

Then you can embed the typeform and populate the hidden field dynamically with the value from you other form. With the latest embed lib the code will look like this:

<div data-tf-widget="<form-id>" data-tf-hidden="email=<value>"></div>
<script src="//embed.typeform.com/next/embed.js"></script>

Alternatively you could read the param from your page URL. Fore example if you redirect users to a page like /signup.html?email=user@example.com you can use this snippet to pass the email to your form:

<div data-tf-widget="<form-id>" data-tf-transitive-search-params="email"></div>
<script src="//embed.typeform.com/next/embed.js"></script>