The replyTo property is strongly typed in the Graph so you just need to use that property in you request eg
{
"message": {
"subject": "Meet for lunch?",
"body": {
"contentType": "Text",
"content": "The new cafeteria is open."
},
"toRecipients": [
{
"emailAddress": {
"address": "target@domain.com"
}
}
],
"replyTo": [
{
"emailAddress": {
"address": "replyto@domain.com"
}
}
]
}
}