Does the very first action in the story have to be the two people meeting online?
No, and for many of you, it probably won't be. Some of you may have your rapist meticulously plan things, research their victim, the whole shebang. Others may just have them randomly meet in a chatroom. Also, some of you may employ flashbacks, and the first meet may be explained there. Just as long as the first time they meet and talk is online, it'll count.
Can I have them meet on the Dark Web?
This is one I have been tossing up since I came up with the idea. Some of us (myself included) have written stories involving the Dark Web, although I don't think any of us know what it is or how to access it, or only know "bits".
For this contest, I will allow meeting on the Dark Web, though you may have to go into detail as to what the victim is doing there in the first place.
Do they need to interact online on that first meeting?
Yes. That first interaction needs to be online.