I assume the food will be put in the microwave before he/she gets home. Otherwise he/she could be asked after getting home.
Student or Learner
a. Do you want me to put some food in the microwave for you when you get home?
My feeling is that technically it can be read in two ways:
1. When you come home, I will put some food in the microwave for you, if you want me to do so. Do you want me to do so?
2. Do you want me to put some food in the microwave for when you get home?
I'd suppose (a) to mean (2) in most cases. But it could mean (1), at least in theory.
Would you say that is right?
Technically, I think it can be read two ways, but it's not much of an offer if the food isn't waiting in the microwave.
If it was supposed to mean that the food would be ready and waiting on the person's return, I would expect it to end "... in the microwave for when you get home". For me, "... for you when you get home" means "When you get home, I will put the food in the microwave".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.