Here are some Mplus example scripts to demonstrate how to set up a missing by design analysis (missingbydesignexample.inp)
along with a data file example (mbdexample.txt). In addition, I've also outlined some of the steps you would use to estimate power for such designs (PowerMissingByDesign.xlsx, missingbydesign.inp ) and the possible deleterious effects of mis-specifying the form of growth over time (missingbydesignlinear.inp).