I would say at this point you need to come clean with your PO and the sooner the better--don't wait until you hand the cup to the technician. You need to be able to establish trust with your PO and saying "oopsie!," when you get caught is probably not going to get that started on the right foot. Come clean, and start off new o. The right foot. And stay clean. It will go better on you now and into your future.
Generally, you are better off being straight with your probation officer. He's giving you the test, so he will know the result soon enough. If you are honest, it will show a sign of taking responsibility. If you don't get clean, you'll just keep getting in trouble. If you can't abstain from marijuana, drugs, tobacco, or alcohol, ask your probation officer about substance abuse treatment or services.
If this test is being done because of a new condition to refrain from using drugs, usually the probation won't be worried about failing the first test. They'll use the results of this first test as a "baseline" to compare against future tests to make sure that you've stopped using (the results should go down each time you test to show this).
It's a violation of your probation whether or not you confess. I think if someone called me and asked me this question, I'd ask what the probation is for, which court it's out of, what their relationship with their PO is like, etc. Is this is a juvenile or adult probation? And how have you done otherwise?
It can be difficult for the DA's office to prove drug tests on a juvenile motion to modify probation, for example, and an admission pretty much does their job for them. But then, it's good to be open with your P.O.
Personally, I wouldn't advise you to make an admission that could result in revocation or placement. It's too vague to give real advice on this.
The first urine analysis is usually used for a baseline to make sure that the level of THC in your system is decreasing as your PO continues to drug test you throughout your probation. So as long as the levels have decreased/zeroed out after your next drug test, you should be fine.