Go to your Abilities tab, press "Ctrl + F" and type "Orb of Lightning". It will lead you straight to "Orb of Lightning (new)". In the Data - Effect Ability, enter the effect you want. In the fields Data - Chance to hit Heroes/Summons/Units, type the chance you want the effect to occur. If you type 45 in the field Data - Chance to hit Heroes, your unit will have a 45% chance to affect the target enemy hero with the effect you've entered via the Data - Effect Ability field.
Finally, the Stats - Duration field will define the duration of the ability effect you want, if it's a debuff, e.g. Cripple.
The Ability you are going to use has to be modified; it must not have any Techtree - Requirements listed.