I'll sum up all these responses and give you exactly what you should do -
You have your account with 19s. And your have you have your veteran account.
Both accounts must have running game time to be able to transfer between them.
You DO NOT need to buy game time for the "f2p" account if its a Veteran account already (meaning you bought the game once already on that acc and you dont have game time ATM)
You can simply ask a GM for a day of game time. There are threads about how to do this. PM me if you have questions.
If the account is a fresh new account, never bought anything. Then you will have to buy the game (which will come with a free month). Just regular WoW no need for BFA.
You can transffer your veteran to that account now that both accounts have game time.
IF you went with the new account route. You can trasffer game time from your fresh new account to your 19s account via a GM. So you get to play your veteran account and add more subscription time to your 19 account.
It's not complicated. Don't overthink it.