Watch the car pull around to the window and wait for the cashier to hand the food bag out the window
While the bag is being exchanged you run inbetween the car and the window, snatch the bag, and run to a secret hiding spot where you can enjoy your ganked food
This can be done with no harm to a stranger as the car's order will be remade for them. It can also be done in league with a friend as you can select exactly what food you want and have that order doubled (but you have to act surprised or pisst in the car; and maybe have the passenger get out and chase the thief to make it seem believable)
Remember, many drive-thrus have cameras so its suggested to conceal your identity with a mask of some sort; the HAMBURGLAR costume will suffice
Driver: What the FUCK?! Freak just Stole my food!
Cashier: No, thats the Hamburglar, He does a fast food handoff here every now and then, we'll have to remake your order.