Not legal in Standard, Alchemy, Standard Brawl, Premodern, Pre-EDH, Old School, Penny Dreadful
Notes & Rules Information
Addendum abilities of instant spells apply while the spell is resolving, not immediately after casting it. If the spell is countered, you don't get the addendum bonus.
(2024-01-12)
If an effect copies a spell with an addendum ability while it's on the stack, the copy wasn't cast at all, so you won't get the addendum bonus.