The defend command will reduce the damage based on percent, e.g. if the defend rate is 50, the combatant will only take 50 % of the damage.
To achieve a similar effect, you'll have to set up a status effect that e.g. increases the combatant's defence status value. The effect would end after Turn Start to remove defend when the combatant selects/starts the next action.
Your defend ability would simply add that status effect.