How do you assign a subaccount to a bill programatically?

I'm scripting the creation of a bill, and I'd like to assign a subaccount of accounts payable to the bill.  This is easy enough through the normal user interface, but on closer inspection, I see a hidden field that references indx_account2.  Trying to set the account with the correct internal id in my script results in the parent account being used. In other words, if the parent account was 800 and the subaccount 801, then if I use 801, the bill is saved with 800.  Is there a way to assign an accounting subaccount to a new bill programatically?

Thanks.