You need two skills on your horse:
First you will want to set a keybind for Evasions (WW) in your Settings > Action Hotkeys. Setting a hotkey to this makes it much easier to use the drift combo because you don’t have to press [W] after drifting and is less key inputs.
Now you can hold the button you assigned, and then drift by pressing [A/D] + [S] and press [F] to cancel the drift and get a burst of speed. Wait a until the horse slows down again to use the combo and repeat.
You can also use the combo without changing your keybinds but this involves pressing [A/D] + [S] and then [W] + [F] together, which means your middle finger has to move between [W] and [S] each time, so it’s good to get into the habit of using the keybind method instead.
This combo can be used to make your horse much faster. You can also use the Charge skill [Q] whilst stationary and then go straight into drift + instant accel.