Got at least one data fetching method working; turns out, we can't use a patched LogicStack to get the data

Vendor/EmbedIO-3.5.2/WebApi/FormDataAttribute.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Collections.Specialized;
+using System.Threading.Tasks;
+
+namespace EmbedIO.WebApi
+{
+    /// <summary>
+    /// <para>Specifies that a parameter of a controller method will receive a <see cref="NameValueCollection"/>
+    /// of HTML form data, obtained by deserializing a request body with a content type
+    /// of <c>application/x-www-form-urlencoded</c>.</para>
+    /// <para>The received collection will be read-only.</para>
+    /// <para>This class cannot be inherited.</para>
+    /// </summary>
+    /// <seealso cref="Attribute" />
+    /// <seealso cref="IRequestDataAttribute{TController,TData}" />
+    [AttributeUsage(AttributeTargets.Parameter)]
+    public sealed class FormDataAttribute : Attribute, IRequestDataAttribute<WebApiController, NameValueCollection>
+    {
+        /// <inheritdoc />
+        public Task<NameValueCollection?> GetRequestDataAsync(WebApiController controller, string parameterName)
+            => controller.HttpContext.GetRequestFormDataAsync();
+    }
+}