Congratulations on finding someone that you want to randomly bang. Anonymous sex can be hot. Just because you are living with HIV, doesn't mean you should not be banned from those fun and kinky sexual endeavors, right? However, depending on the state that you are living or country, disclosing your HIV status prior to sexual contact is the law or you could face time behind bars. And who has time for that, right? However, you should also know that being able to prove that you disclosed is what could keep you out of jail.
First, you need to be very aware of the laws where you live about disclosure of HIV prior to sexual contact. If you live in the United States, the Centers for Disease Control offers a state map with what the specific legal requires are for that state. Whether or not we agree if those laws are fair to those living with HIV, you have to follow the law.
Second, although you realize that the actual risk of transmitting HIV is very low, your potential sex partner may not know that undetectable means that you are a 'near zero' transmission risk. So, before they are balls-deep in you (or before you are balls deep in them), you must act in an ethical manner and given them the option of whether they want to make you scream in bed like a little bitch. It's simply the right thing to do to give them that opportunity.
Finally, nobody knows the best time to disclosure an HIV positive status. It really is up to the person. Here is my suggestion, and how I navigate sex:
If they say they are not cool with it, don't get mad. Say thank you for being honest and then block them and look elsewhere. Sex is supposed to fun. Hiding or lying is not fun!
If you both make it past step four, have fun!