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

Contexts/SecureBuffer.cs

@@ -0,0 +1,20 @@
+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 SecureBuffer
+    {
+        public int Count;
+
+        public BufferType Type;
+
+        // A pointer to a byte[]
+        public IntPtr Buffer;
+    }
+}