You have to know which spell you based the custom spell off. For example a custom AoE spell based off Blizzard.
In triggers, you "Issue order". It's an action in the unit menu. There are many types of issue order actions - for example issue order targeting a point has all spells that target point (= not unit) - e.g. Flamestrike. Then there is issue order targeting a unit - e.g. Fireball. Last is issue order with no target - E.g. Starfall.
Now you order your unit to cast the spell you based your custom spell off. So if your spell is based off Flamestrike, you order the unit to cast Flamestrike. Flamestrke is a point-target ability, so you won't find it anywhere else. That means you cannot order a unit to cast Flamestrike on specific unit. Same as you cannot order Fireball spell to be cast on a location, etc.