No Guarantee
You can't guarantee a dapple grey foal from any specific color combination.
Why
* Dapple grey is a pattern, not a base color. It's a dilution of bay, chestnut, or black.
* The grey gene is dominant, so any horse with it will be grey.
* The dapple pattern is unpredictable.
Improving the Odds
Breed a grey stallion to a mare with a dark base coat or one with dapple lineage. Horse color genetics are complex and unpredictable.