you try to come up with a situation in which you could safely fox trot into an fsmash. keep in mind fsmash takes 10 frames to come out on top of however many frames marth's foxtrot takes to start, move, and end.
only thing i can think of is you are 2 dash lengths away from snake and he decides to try an fsmash in the hopes you will dash all the way up to his face and get hit by it. or your opponent is recovering and you know for one reason or another that they will miss the ledge and land on stage with enough lag that you will be able to fsmash but are at that moment too far away. bearing in mind that foxtrots have a set distance, meaning that even if these situations arise, the positioning needs to be favorable.
the other option is foxtrotting into a pivot smash as a defensive move to hit where you were given the expectation that your opponent will move into the area after you dash away from it. in most situations where you are retreating as an offensive and defensive maneuver, dash pivot db or boosted pivot grabs tend to be better options by far.
the odds that you will be in a situation where dashing into an fsmash is your best option is quite low.