<form action="https://api.unisender.com/ru/api/subscribe" method="get">
<input type="hidden" name="format" value="json">
<input type="hidden" name="api_key" value="ВАШ_API_КЛЮЧ">
<!-- ... -->
<input type="text" name="fields[email]">
<input type="text" name="fields[name]">
<!-- ... -->
<button type="submit">submit</button>
</form>