Found when updating an app from an rc.2 build to latest rtm build.

Info

SDK version: 8.0.100-rtm.23471.13

Repro

1. Create a new Blazor Web App

2. Update the Home.razor to the following:

   @page "/"
   
   <PageTitle>Home</PageTitle>
   
   <h1>Hello, world!</h1>
   
   Welcome to your new app.
   
   @if (items is not null)
   {
       @foreach (var item in items)
       {
           <form method="post" @formname="@($"addtocart-{item.Id}")" @onsubmit="HandleAddToCart">
               <input type="hidden" name="ItemId" value="@item.Id" />
               <input type="hidden" name="ItemName" value="@item.Name" />
               <AntiforgeryToken />
   
               <strong>@item.Name</strong> <button type="submit">Add to cart</button>
           </form>
       }
   }
   else
   {
       <p>Loading...</p>
   }
   
   @if (!string.IsNullOrEmpty(message))
   {
       <p>@message</p>
   }
   
   @code {
       private List<Item>? items;
       private string? message;
   
       [SupplyParameterFromForm]
       public int? ItemId { get; set; }
   
       [SupplyParameterFromForm]
       public string? ItemName { get; set; }
   
       protected override void OnInitialized()
       {
           // Load items
           items = [new(1, "Item 1"), new(2, "Item 2"), new(3, "Item 3")];
       }
   
       private void HandleAddToCart()
       {
           message = $"Added {ItemName} with id {ItemId} to cart";
       }
   
       record Item(int Id, string Name);
   }

3. Run the app and try to click one of the "Add to cart" buttons

Expected

The form can be submitted successfully and the form handler runs.

Actual

An error message is shown in the browser:

The POST request does not specify which form is being submitted. To fix this, ensure <form> elements have a @formname attribute with any unique value, or pass a FormName parameter if using <EditForm>.