Unfortunately...

I'd much rather have it (and trinkets) fire off, on the macro that it was put into.
But that's not how the game works.

Personally, I've just said screw it, it will fire off whenever it is ready.
And I'll have maximum up-time on the ability.

But I'd rather have an ability like that, when I choose for it to fire off.
But also not sure its worth an extra thing to manually control.