Ah I see. Two ways, from my view.
The first is through the work experience section. Just putting a short note saying that it’s yet to commence. If you know of any seats you are going to be completing, it’s worth mentioning. If these are core practices of the firm you’ll be completing a vac scheme at, then that’s a great link to make. However, the question in any recruiters mind will then be why are you applying to them if you have a vac scheme coming up. This will be more so at interview stage, however, where you are asked that explicitly. So you’ll need to have solid answers prepared. Another thing is that the word count may not be enough to blab on about so much detail in this section. In the work experience section as well, you’d rather keep this away from any tangents. In summary, a simple “yet to commence” note, together with some supplementary detail you deem necessary, should suffice.
The second way to talk about it is in the why commercial law section / cover letter. Simply saying something like your incoming vacation scheme will further expose you to the profession. Though, there’s less to talk about here too since it’s incoming and not yet completed. Nevertheless, including it, however so slightly, still beefs up the quality of your application.