Sending the first message is the part most people get stuck on. You are writing to somebody you have never met, you have no idea how they will take it, and there is no tone of voice to help you out. The good news is that it does not have to be clever. It has to be short, warm, and clearly meant for that person rather than for anybody at all. Five things make the difference.
"Hi, how are you?" gets deleted, because it could have been sent to anyone. Read the profile properly and pick one thing to respond to. A photo taken somewhere you recognise, a hobby they mention, a book on their list. Something like "I see you do the parkrun at the dam on Saturdays. How long have you been going? I keep saying I will start and then Saturday arrives." A message like that could only have been written to that one person, and that is what earns a reply.
Three or four sentences is plenty. A long first message is a lot to answer, and it spends everything you might have talked about later. Say hello, mention the thing you noticed, ask one question, sign off with your name. There is no prize for filling the box, and a short message is easier on the person reading it on their phone.
A message made up entirely of questions reads like an interview. Offer a line about yourself so they have something to pick up on. One sentence will do it: what you did over the weekend, why their photo caught your eye, what you are halfway through reading. You know yourself well by this stage of life, which makes this a good deal easier than it was at twenty five.
A bit of humour works well, and a light touch is best in writing, where there is no tone of voice to carry it. Gentle, and slightly at your own expense, is the safest ground. Teasing works between people who know each other and lands very differently coming from a stranger in writing. Steer clear of jokes about their looks, and anything off colour in a first message. If you are not sure how a line will read, leave it out.
End with one open question they can reply to without much effort. "Whereabouts are you from originally?" works far better than "Tell me about yourself", which is a big ask of somebody who does not know you. In South Africa that first question tends to be a good one, because so many people have moved province at some point and most have a story attached to it. One question, though, not five.
A few last things worth doing. Read it back before you send it, because typos are easy to make on a phone. Use their name if they have given one. And do not send the same message to twenty people, because it shows. If you do not hear back, that is normal and it is rarely anything to do with you. Move on to the next one, and keep it just as personal.