anyshape

<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>

document.querySelector('img').addEventListener('load', function() {
            alert(`width = ${ this.naturalWidth }px\nheight = ${ this.naturalHeight }px`)
        })