Pre-paid Telstra credit?

So I just changed to Telstra and I'm wondering what I do about getting credit. If I just ask for $30 credit, and then call the number thing on it, will it just be put as $30 normal credit or will there be options for the pre-paid things like the Pre-paid cap which is what I want? Just wondering because I've never been with Telstra before and I want to make sure I get the Pre-paid cap with the bonus credit.. ??