Project project = new Project("New Project.mpp");
Task task = project.RootTask.Children.GetById(1);
Resource resource = project.Resources.GetById(1);

// Create resource assignment
ResourceAssignment assignment = project.ResourceAssignments.Add(task, resource);
assignment.Set(Asn.Notes, "Newly added assignment");

project.Save("UpdateResourceAssignment_out.mpp", SaveFileFormat.MPP);