When you enable Crypto Plus trading permissions, you are required to have USD in your Cryptocurrency Service Providers account to fund your crypto buy trades. You "pre-fund" your account by transferring money from your linked IBKR account within the Cryptocurrency Service Providers web app.
If you attempt to submit a cryptocurrency order and either have NOT pre-funded or are short funds, you will receive a rejection and will not be able to submit the order until you have transferred sufficient funds.
All exchange and custody services in connection with trading and holding cryptocurrency tokens and other digital asset tokens through IBKR trading platforms are provided by Cryptocurrency Service Providers. Learn more.