Opening Multiple ISA's today and now changed mind. How much can I invest?

Good evening, I opened a Flexible ISA just now and was about to transfer in a full £20000 tonight and then my partner mentioned that they had a Moneybox ISA, which is NOT flexible, but that they are offering a chance to win a portion of a £50k pool of money! So I have now changed my mind and want to open a MoneyBox ISA and put the £20000 in there instead. Could I technically put the £20k into the Flexible ISA and have a bit of a cooling off period and then transfer out the £20k from there into the MoneyBox ISA before 5th April next? Or is it safer to avoid confusion for HMRC to simply put the money straight into the MoneyBox ISA instead of the above convoluted way? So typical of me to change my mind at the last minute. Chances of winning with the MoneyBox ISA are probably near to zero but why not?

Hi,
You can see guidance here:
Individual Savings Accounts (ISAs)
We cannot advise you on where to invest your money as this is financial advice which we are not authorised to give.
Thank you.