The magic isn't so much in the cue itself...you use whatever pleases the horse and keeps you safe. That means, you probably shouldn't start it while standing in front of the horse, but rather from beside the horse.
Horses can easily learn to target something with the front legs...so you walk beside the horse and hold the target close enough to the front legs that the horse accidentally bumps it. Stop and praise enthusiastically (even though it was an accident...
). If you use food rewards (or click and treat) click (or mark with the voice) the very second the horse touches the target. Then repeat, repeat, repeat until you notice the horse TRYING to touch the target. They are smart...they figure these things out.
Because you are walking or standing beside the horse, a longer target is good. This can be a whip, or you can start with something softer, that the horse doesn't mind bumping with a foreleg. I used a pool noodle.
As the horse figures out to raise the legs, you add a verbal cue (I use, "step"). If you don't wish to add a verbal cue, that's fine...you'll find that your horse will read your body language and at least from the ground, when you are in the position you are normally in when you ask for Spanish Walk, and reach over AS IF you had the target in your hand, your horse will offer Spanish Walk.
So for some people the cue may become pointing at the front legs. For me, I hold my hand and arm out as if I'm holding the pool noodle for him to touch.
There is no one correct cue for it. You can just play and let your horse choose the cue!