On the retail side Fidelity and TradeStation have done this. Goldman and others are also in https://www.coindesk.com/goldman-sachs-to-enter-crypto-market-soon-with-custody-play-source Any chance IB joins this soon? Set margin requirements at 100% (like OTC stocks), I dont mind, just allow people to buy and custody the main cryptos, please
Open an account at tradestation crypto and then transfer securities from IB there. Either sell the securities and buy crypto or buy on margin. Otherwise trade CME or Bakkt Bitcoin futures at IB. Also microstrategy stock and options are a bitcoin proxy as well as gbtc.
This specific broker does not allow for direct buying and selling of cryptocurrency. There is an option of trading bitcoin futures which is like any ordinary future contract such as stocks, indices, etc. The risk is very high in the bitcoin futures so the other thing which you can do is that sell the security in IB and invest the returns in cryptocurrency through some other broker.