Thank you!You can always put your future role on an application form anyway, you just won’t be able to provide lots of detail about it - but I would still expect that to be the case if you are only a few weeks into a role.
For a cover letter:
Why the firm
Why the opportunity/why commercial law
Why you
Ideally if you can get it on one page, great, but no more than a page and a half or about 750 words