Can you please share a screenshot of the request made from MS Graph Explorer?
Following is working fine for me:
POST https://graph.microsoft.com/v1.0/me/sendMail
Content-type: application/json
{
"message": {
"subject": "Meet for lunch?",
"body": {
"contentType": "Text",
"content": "ENTER_URL_HERE"
},
"toRecipients": [
{
"emailAddress": {
"address": "frannis@contoso.onmicrosoft.com"
}
}
]
}
}