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You do not need to have IDR in your wallet, what you need to do is to change the "in-game" currency when you open the game or when the game is already opened.
At stake, once you open the game then you will see an option written as "balance displayed in:" simply click it and scroll till you find IDR or any other currency you wish to be used.
Take a look at the image below to make it clearer:

Or if you have opened the game, this option is still available under the game next to the other option/menu "open in mini player"
Thanks, @panjul07! I didn't know about that. And I prefer making the smallest bets possible, I thought it'll give the opportunity to bet with less. But apparently that's not the case. After changing my currency, as you explained, I found out that the minimum bet I could make was

, and that's 0.024 USD, while I can bet with as little as 0.01 USD on the same Pragmatic slot. (It was new to me, btw, because I always thought the minimum on Pragmatic slots was $0.20). But don't bother, I really don't want to bet with less than 1 cent, bc I think it's not interesting.