// Set constraint Must Finish On for task with Id 15
Task task = project.RootTask.Children.GetById(15);
task.Set(Tsk.ConstraintType, ConstraintType.MustFinishOn);
task.Set(Tsk.ConstraintDate, new DateTime(2017, 3, 1, 18, 0, 0));

SaveOptions options = new PdfSaveOptions();
options.StartDate = project.Get(Prj.StartDate);
options.Timescale = Timescale.ThirdsOfMonths;
project.Save("MustFinishOn_out.pdf", options);