State storage - form

// *** This goes to Program.cs - use static file middleware ***
app.UseStaticFiles();

// *** Create wwwroot folder in project and a new index.html file in it. ***
// *** This goes to index.html ***
<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8" />
    <title>Form example - receipt</title>
</head>
<body>
    <form method="post" action="https://localhost:7246/api/Receipt/form">
        <input type="hidden" name="ReceiptId" value="12" />
        <select name="ReceiptTitle">
            <option value="">(select subscription)</option>
            <option value="0">Monthly Subscription</option>
            <option value="1">Monthly Subscription - Family</option>
            <option value="2">Quarterly Subscription</option>
            <option value="3">Annual Subscription</option>
        </select>
        <br />
        <input type="text" name="ReceiptTotal" placeholder="Total" />
        <br />
        <input type="submit" value="Update data" />
    </form>
</body>
</html>

// *** Add the model for the form handler ***
public class ReceiptFormData
{
    public int ReceiptId { get; set; }
    public int? ReceiptTitle { get; set; }
    public decimal? ReceiptTotal { get; set; }
}

// *** Add the form handler to the controller ***
[HttpPost("[action]")]
public ActionResult<Receipt> Form([FromForm] ReceiptFormData data)
{
    var titles = new string[] { "Monthly Subscription", "Monthly Subscription - Family", "Quarterly Subscription", "Annual Subscription" };

    var retVal = _receipts.FirstOrDefault(x => x.Id == data.ReceiptId);
    if (retVal == null)
        return NotFound();

    if(data.ReceiptTitle.HasValue)
        retVal.Title = titles[data.ReceiptTitle.Value];

    if (data.ReceiptTotal.HasValue)
        retVal.Total = data.ReceiptTotal.Value;

    return Redirect("/index.html");
}