Some of the most useful things people do are small. They clarify a need, make a careful introduction, answer a question, or tell you honestly that a job is outside their scope. Those actions deserve to be noticed.
Choose one action and say why it mattered. ‘You explained the timing before introducing us, so we could have a useful first conversation’ tells someone what you value. It does not need to become a long message.
Keep a private moment private. Ask before using someone’s name, words, or client story in a public post. A thank-you should not turn into an endorsement the person never agreed to give.
Resist attaching a request. You can ask for business in another conversation when there is a reason. Let this message be about the work already done and the person who did it.
Notice the habits you appreciate and practice them yourself. Clear context, honest answers, and a finished next step are things other people can feel in a working relationship.