Create a Kissend account with email or Google. Pick a username that is not your phone number and not your full legal name. Send that username to the person, or ask for theirs.
Accept the friend request. The direct chat is free. If you are on a shared phone, turn on the app lock before the thread gets personal.
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Do not post the username in a public place if the point was to stay hard to find. Search can find a username, and it can also match the name on your profile.
Do not use the website's meet-someone text chat for this. That mode introduces a stranger. A friend request is the path when you already know who you want.