I'd think if you write a complete message, introducing yourself, saying what you appreciated about them and think you'd enjoy about playing with them, and saying that if they are interested you'd love to talk more and see if something might work that would probably be well received. My guess is that the people who are bombarded by these requests get one line, text-speak messages asking for them to be the dom/me to the sender.
If their signature or profile says "don't ask to be my sub" then I wouldn't ask them though
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