How do I transfer funds to NIB custody account?

I have an NIB current account.

I wish to buy shares online, and have just got a custody account.

So on my computer screen, I can see my existing account and the new (empty) custody account.

My question is: how do I transfer cash from the current account into the custody account, so as to buy shares?

Or do I just buy the shares, and the system takes the cash from my NIB current account?

I believe that you purchase the shares and identify your current account as the source of funds.
Custodian account holds shares not cash.

regards
Paddy