The sample project has a couple of examples of this working. The default block effect causes the guard motion when the shield animation pops up.
The Parry one uses the attack motion, and specifically calls for it using the motionRequested variable in the codebox.
//makes a melee swing motionRequested = "attack" value = Ramza.SB.make_damage_value.call(this, target, critical) value = this.applyBlockDamageModifier(target, value)
Make sure that the motion request is in quotes as above, so the system knows it's a string.
~Ramza