Because exiling a card with foretell from your hand is a special action, you can do so any time you have priority during your turn, including in response to spells and abilities
The first thing to note is that players dont receive priority in this step , unless one (or more) of the automatic actions detailed below happens to trigger an effect and puts it on the stack it's only in that case that the active player gets priority to react, and other players can respond
Illustrated by Michael Ryan Standard Not Legal Alchemy Not Legal Pioneer Not Legal Historic Not Legal Modern Legal Brawl Not Legal Legacy Legal Comp
That's as much a WotC problem as it is a player availability problem, but for now it's safe to say Warhammer s uniqueness and the fact that the cards are unlikely to see proper reprints means price spikes like this might continue happening
I probably dont need to explain why discarding cards to draw cards is the best way to take advantage of whenever you discard triggers, but Id argue its so good and central to the plan that you should basically consider whenever you draw effects to belong in the discard archetype as well