in the action list it is one of these:
"Unit - Issue order targeting a unit/a point/a destructible/an item"
or
"Unit - Issue order with no target"
If the ability targets a unit (e.g. Holy Light), you want to use the action "Issue order targeting a unit"; if it is targeting a point (e.g. Blizzard), you want to issue order targeting a point; etc.
If it does not require any target (e.g. Starfall, Berserk, ...), you have to "Issue order with no target"
Also, you have to issue an ability from which you based off your own custom ability.
For example - If you create new ability called "Passive heal", which is based off Tranquility, and you want a dummy unit to cast this "Passive heal" ability, then you have to issue order with no target -> in the list find "Night Elf - Tranquility" (or something like that); You have to order Tranquility because your own ability is based off tranquility.
If you based off your ability off Blizzard, then you have to issue "Blizzard", etc.