// Set constraint Must Start On for task with Id 5
Task roof = project.RootTask.Children.GetById(5);
roof.Set(Tsk.ConstraintType, ConstraintType.MustStartOn);
roof.Set(Tsk.ConstraintDate, new DateTime(2017, 1, 1, 9, 0, 0));

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