I can now successfully call InitializeSecurityContext and get a status of ContinueNeeded.

Contexts/SecureBufferDesc.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Runtime.InteropServices;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace NSspi.Contexts
+{
+    [StructLayout( LayoutKind.Sequential)]
+    public unsafe struct SecureBufferDesc
+    {
+        public int Version;
+        public int NumBuffers;
+
+        // A pointer to a SecureBuffer[]
+        public IntPtr Buffers;
+
+        public const int ApiVersion = 0;
+    }
+
+
+}