HTML code

<form action="/a" method="POST">
<input
    type="text"
    value="String"
    name="string"
    id="filename"

/>

<input type="submit" value="Submit" />
</form>
<script>
document.querySelector("form").addEventListener("submit", (e) => {
    e.preventDefault();
    const string = document.querySelector("#filename").value;
    const element = document.createElement("a");
    const file = new Blob([string], { type: "text/plain" });
    element.href = URL.createObjectURL(file);
    element.download = "test.txt";
    document.body.appendChild(element); // Required for this to work in FireFox
    element.click();
});
</script>