The signature is a feature which belongs Outlook online. The Office 365 REST API only provide the mail service. However you can set the message content contains the signature in the app as you want.
For example, here is an example that use the Html format to set the message content contains the signature:
{
"subject":"Test signature",
"body":{"contentType":"html","content":"<html><head></head><body><div>Your Message content</div><div>Your signature</div></body></html>"},
"toRecipients":[{"emailAddress":{"name":"user1","address":"[email protected]"}}]
}