solution for .NET 8

define the following class:

internal class UnsafeAccessorClassAntiforgeryOptions
{
    [UnsafeAccessor(UnsafeAccessorKind.StaticField, Name = "DefaultCookiePrefix")]
    public static extern ref string GetUnsafeStaticFieldDefaultCookiePrefix(AntiforgeryOptions obj);
}

then in Program.cs as the first line:

UnsafeAccessorClassAntiforgeryOptions.GetUnsafeStaticFieldDefaultCookiePrefix(new()) = ".AntiForgery.";

more info about UnsafeAccessorAttribute at:

https://medium.com/@malarsharmila/introduction-to-the-unsafeaccessorattribute-class-in-net-8-0-d3a55ec15762