Your available settled cash balance in the currency of denomination of the product will be used before opening a margin loan.
Margin is configured on an aggregate basis, so although you can fully pay for stocks in a margin account, once you run out of cash to pay for the securities IBKR will extend a loan.
There is not a way to pay for some stocks and take a loan on others without affecting the account as a whole. Please note that a broker who finances client purchases of securities via margin loan is allowed by regulation to loan or pledge as collateral that client’s securities in an amount up to 140% of the cash debit balance. Any securities held by the client in excess of that amount are referred to as excess margin securities and are required to be segregated unless the client provides IBKR the authorization to lend through the Stock Yield Enhancement Program.