index.html

<!doctype html>
<html lang="ja">
<head>
    <meta charset="UTF-8">
    <title>Open Redirect Demo</title>
    <script defer>
        const u = new URL(location.href)
        const searchParams = u.searchParams;
        document.addEventListener("DOMContentLoaded", () => {
            if (searchParams.has("redirect_to")) {
                const redirectTo = searchParams.get("redirect_to");
                if (confirm("Redirecting:" + redirectTo)) {
                    location.href = redirectTo
                }
            }

            document.querySelector("input").addEventListener('input', (e) => {
                const value = e.currentTarget.value
                const a = document.querySelector("a")
                if (value) {
                    const newUrl = new URL(location.href)
                    newUrl.searchParams.append("redirect_to", value)
                    a.href = newUrl.toString()
                } else {
                    a.removeAttribute("href")
                }
            })
        })
    </script>
</head>
<body>
    <label>
        Redirect to:
        <input type="text" />
    </label>
    <div>
        <a>open</a>
    </div>
</body>
</html>