DemandOnPayment function
demandForPayment( responseBack, username, password, payer, payee, txn )
| Name | Type | Description |
| responseBack | Indicates which function to be called when a response is received. | |
| username | The username should normally be the msisdn in addition to its country code i.e. +18881234567 | |
| password | The user password | |
| payer | The Customer object which will be making the payment | |
| payee | The Customer object that will be receiving the payment | |
| txn | The TxnData object that contains txn details |