Answer
Header
It needs to grab the attention, stand out, and be in line with the content of the page where your pop-up is displayed.
Input fields
Fewer fields — higher conversion. This is what most email marketers agree on.
Button
You need an easy-to-spot button to grab attention with a call-to-action text. For example, “Subscribe” or “Get it now”. The CTA text needs to explain clearly what users will get.