I found the answer to this question. Just add the field <input>to <form>and get the value from this input field on your PHP page, for example:
$input = $_POST["inputvalue"];
Thus, the form will look like this:
<form action="/api/stripe/charge" method="POST">
<script src="https://checkout.stripe.com/checkout.js"
class="stripe-button"
data-key="pk_V4y6c4urwnTkLMEVNs0qBIQQJ5Yzu"
data-image="/square-image.png"
data-name="Demo Site"
data-description="2 widgets ($20.00)"
data-amount="2000"
data-currency="usd"
data-bitcoin="true">
</script>
<input type="hidden" name="inputvalue" value="value">
</form>
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