Board Game Arena

So I have a surprise card that allows me to take a Keeper that another player just puts down. But I don't know how to use it. I see the button to retain surprise but I want to use it to see the keeper that was just put down. I click on my surprise card and then the keeper in the surprise queue but nothing happens.

I suppose you figured this out already meanwhile, but just for completeness:
you just click the Surprise card in your hand like you did, whenever the situation allows it.
If you don’t immediately get the result but it (or in this case the Keeper) gets added to the Surprise queue, that just means that other players could possibly still play other Surprises to cancel out yours.