Yep, this did it. H E R O
A puzzling number of quotes and double quotes, but thanks so much for your insight!

For those wondering, here's my updated Send command:

With CreateObject("MSXML2.XMLHTTP")
            .Open "POST", sURLNotes, False
            .Send "{ ""is_todo"": 1, ""title"": """ & objItem.ConversationTopic & attachmentName & """" _
            & ", ""parent_id"": """ & sFolderID & """" _
            & ", ""body_html"": """ & sEscapedBody & """" _
            & " }"
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