Depends on which academy you are attending. USAF Academy? Westpoint?
I would expect it to show on your DEC LES statement. Most likely, it's 0-1. But that's just a guess because it is perfectly possible for a cadet to start the academy as an 0-2 or higher, depending on their background experience and what course of study is being pursued.
If anything, the local paymaster can definitely answer this question for you.
Some of the others here have been recommending using E4 or E5, the grade you might end up at if you wash out. Certainly, your monthly pay is less than an E1 and nowhere near any officer (I think LES forms show C1 to C4 as your grade).
The historical intention that TTX appears to have had, was to allow the lower-paid enlisted grades use the Deluxe pay-for-it version for free....this year (for 2019) they've upgraded it to any enlisted (Even E9 !)..any version (but not LIVE).
But if you are a Cadet, at any of the main US Military Academies, you should see if there is an Academy-sponsored tax Drop-Off service (West Point has one)...they know the subtleties of Cadet tax submissions (you are not in college, unless you are in a college ROTC program).
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Or were you already enlisted, and then later accepted into the particular US academy you are in (yeah, that does happen)....then just use whatever E-grade listed on your LES, or if it's been changed to a C-grade, still, I'd use your last E-grade.