I personally wouldn't tell someone right off the bat. I just feel like that's something personal, and saying anything too personal right away can put someone off. Instead, I would bring it up in a conversation after you get to know them a little better. For example, the way I brought it up with my last boyfriend was to say something like, "Do you ever feel really anxious? Like you're panicking?" Most people will say yes, and then you can explain your anxiety to them. That opens the door more than just being like, "OH FYI I have generalized anxiety disorder".
That's what I would do
Hope it helps :)