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I have a deck focused around Entreat the Angels. It uses Doubling Cube to mass mana when I pull a miracle cost Entreat the Angels. The problem I am having is I cant do that in forge and just end up playing it for it's hard cost. Here's an example...

I have 2 caged suns out, 10 plains, 2 gilded lotus and 2 doubling cubes. So I tap everything resulting in 36 mana, minus 3 then double for one cube, 66, and once again for the other to get 126 mana which I can then spend on it's miracle cost resulting in 124 6/6 angels.

In forge however I go to tap the mana and it just assumes I want to tap it to pay for it's miracle cost right there. Is there a solution for this? Is there a button I can hold down to tell forge to ignore paying the price and just add it to my mana pool or something?

I don't think we have any way to do this once the cost is already being paid. Best you can do is while the Entreat Trigger is on the stack before it you can start paying the cost, generate all the mana you want for it.