Read the profile before you write. Mention one thing from it. "Hello, how are you?" gives the other person nothing to answer.
Say why you wrote, and ask one question that is easy to reply to. Keep it short. A first message is an invitation to talk, not a biography.
Messaging on Only Dating is free. Stay on the site until you have a reason to move the conversation elsewhere. Anyone who asks you to continue on WhatsApp straight away is worth slowing down for.