Get the impression a lot are doing this wrong. Whether logged in via Mobile View or desktop, you should text at the top saying Inbox. Click on that. This is how it looks on Mobile View
And this is how looks on Desktop
After clicking on that, you’ll come to this screen. Type the users name in the recipient box. It should auto populate, but after typing in you may need to delete a letter or two back for the auto populating to kick in (I don’t know why). Text what you want and then post message. It will then go to the other users Inbox where they access via the same Inbox button
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(someone will be glad the question was asked;)