On this page you can select what crypto currency you want to purchase (in this case BNB), and what local currency you want to pay with. If you have a bank account linked to your Binance.US account, then you can select that as your local currency to pay with that.
After you confirm your transaction, you need to now withdraw the newly purchased BNB from your Binance.US account and move it to your wallet associated with your Featured account.
To do this, first hover on the [Wallet] tab found at the top, then click [Withdraw].
On this page, you need to select BNB from the Coin drop down, then enter you wallet address (the one used for Metamask, Torus, Trust Wallet, or Binance Smart Chain Wallet), a memo, and the amount you want to send. Then click [Submit].
Double check your wallet to be sure the transaction went through properly.