Trying to limit field security to owner of record using PrincipalObjectAttributeAccess

I've got a field on an appointment entity - PrivateNotes. I only want to allow the OWNER of the field to be able to read & update this field. So, field security will not suffice.

I googled this question, and found some code that uses PrincipalObjectAttributeAccess to restrict an individual field's security to just the owner, however I can't get the code to work, and I've tried several ways. PrincipalObjectAttributeAccess is new to me, I never heard of it before today.

Here is the simplest version I found:

       private void ShareSecureFieldWithOwner(Entity record)
        {
            // Any method that helps you find the AttributeMetadata Id
            var attributeId = FindSecuredAttribute();

            if (attributeId != Guid.Empty)
            {
                var userAccess = new PrincipalObjectAttributeAccess
                {
                    AttributeId = attributeId,
                    ObjectId = record.ToEntityReference(),
                    PrincipalId = record.OwnerId,
                    UpdateAccess = true,
                    ReadAccess = true
                };

            context.AddObject(userAccess);
            context.SaveChanges();
            }
        }

2 main problems with this:

1. I don't know how I'm going to implement FindSecuredAttribute.

2. It's telling me that I'm missing a reference or assembly, as it doesn't recognise PrincipalObjectAttributeAccess. I'm trying to find what using statement/namespace/reference I need for this, but can't find anything.

Here are the current using statements I have:

using Microsoft.Xrm.Sdk;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

I REALLY don't know what I'm doing here, so I need the simplest explanantion possible!

Thanks.