Refer to something they actually wrote. A first message that could have been sent to anyone is easy to ignore, and it is also how copy-paste accounts introduce themselves. One specific line from their profile is enough to show you read it.
Be clear about the arrangement you want. Ambiguity wastes both of your time. You do not need to be explicit about anything you would not say to them in person. You do need them to understand whether you are free to meet, how discreet you need to be, and what you are not offering.
Keep it short. A long first message about your history, your marriage, or what you want done is a lot to hand a stranger. Two or three sentences, one question, and stop. Let them answer before you add anything you would not want forwarded.
Do not ask them to continue on another app in the first exchange, and do not send a phone number, a surname, or a photo you cannot take back. Anyone who says the site is being watched, and that you should move, is using a common line. The conversation can stay here.
If their profile is vague, you can ask one plain question about what they want. If they will not answer it, you already have your answer. Do not fill the silence with more personal detail.
A reply that rushes you, flatters you, or asks for money is not a good reply. Read the guide on spotting a fake profile before you send anything identifying.
Create a free account to message someone on Only Amour.