Hagaluly You need to set the connection type as winrm. Also make sure WinRM is running in the machine and firewall allows connection.

provisioner "file" {
  source      = "conf/myapp.conf"
  destination = "C:/App/myapp.conf"

  connection {
    type     = "winrm"
    user     = "Administrator"
    password = "${var.admin_password}"
    host     = "${var.host}"
  }
}