When you're a new customer and you upgrade to a paid account, you'll automatically be issued a new unverified TFN, then you just need to follow the usual process to get it verified. This article can help you with that.
If you're an existing customer and you'd like to add a TFN to your account, you can request one from the Sender IDs page in the Hub. Just click on the blue Get a dedicated number button and follow the prompts and you'll be issued with a new unverified TFN. Then, like above, you just need to follow the usual verification process.
Note - if you're having trouble requesting a new dedicated number via the Hub, you can submit a request and our team can lend a hand.