If you're looking for a script function, it must be this one: AcceptBattlefieldPort

It seems a little trickier than the others with more parameters, but I think you can make it work. There's an example if you follow the link, but it looks a little too big to fit in a script.

edit: Actually, if you are only in one queue, maybe this could work:
/script AcceptBattlefieldPort(1,1)

Otherwise, maybe the index is different for different BGs? You could do an ugly hack: :-D
/script AcceptBattlefieldPort(1,1)
/script AcceptBattlefieldPort(2,1)
/script AcceptBattlefieldPort(3,1)
...