How to get "messageTop" to work with excelHtml5

I need to place a message into the body of an exported excel document and I'm not having any luck. The data table exports just fine, but I don't see the message.

I've tried this.

buttons: [
    {
        extend: 'excelHtml5',
        messageTop:'This is a test'
    }
]

I've also tried this.

buttons: [
    {
        extend: 'excelHtml5',
        exportOptions:
        {
            messageTop: 'Another test' 
        }
    }
]

What am I doing wrong? How to I get message text to appear above the exported table?