If you are using Arduino, just add the following lines in your initialization
XBeeAddress64 addr64 = XBeeAddress64(0xDH, 0xLH); //DH is the MSB address of your address, LH is the LSB address of your router, example XBeeAddress64 addr64 = XBeeAddress64(0x0013A200, 0x40A7DB96);
//create the packet
ZBTxRequest zbTx = ZBTxRequest(addr64, payload, sizeof(payload));
// you can then send the packet as usual
void loop() {
xbee.send(zbTx);
}