See https://jcallaghan.com/2019/10/migrate-sharepoint-discussion-boards-to-yammer/ for more information. #Website

Yammer-CreateMessage-Sample.ps1

function Get-BaererToken() {
    $headers = @{ Authorization=("Bearer " + $token) }
    return $headers
}

# Yammer app developer token
$token = "12345678-ABCDEabcdeABCDEabcdeAB"

# Base Yammer API url
$yammerBaseUrl = "https://www.yammer.com/api/v1"

# Create the json for a new message.
$messageBody = @{
    body = "Video provides a powerful way to help you prove your point. When you click Online Video, you can paste in the embed code for the video you want to add. You can also type a keyword to search online for the video that best fits your document. To make your document look professionally produced, Word provides header, footer, cover page, and text box designs that complement each other"; `
    group_id = "1234567890"; `
    is_rich_text = $false; `
    message_type = "normal";
}

# Post new message to the messages API
$headers = Get-BaererToken
$messagesApi = "$($yammerBaseUrl)/messages"
$apiResponse = Invoke-RestMethod -Uri $messagesApi -Method Post -Headers $headers  -ContentType "application/json" -Body (ConvertTo-Json $messageBody)