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I play Snapcaster Mage and targeted an instant. How do I use that "flash"? Is this something that has been implemented yet?

Are you a Forge user by any chance?

Yes. I'm just wondering if it's possible to pay the cost to use that targeted instant.

Click the icon that looks like a lightning bolt. That's where you should go to cast cards from zones other than your hand (such as from the top of your library, from your graveyard or from exile.)