[android help] Set notify url to paypal payment in my android apps

Normally on websites, we can set a notify URL to check payment with paypal server. However how do we set it in an app? I am super new to paypal app payment.

I am using MEP.PayPal

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    switch(resultCode) {
    case Activity.RESULT_OK:
        String payKey = data.getStringExtra(PayPalActivity.EXTRA_PAY_KEY);
        data.getStringExtra(PayPalActivity.EXTRA_PAY_KEY);
        Toast.makeText(this,"Paymnet Successful : " + payKey,Toast.LENGTH_LONG).show();
        //finish();
        break;
    case Activity.RESULT_CANCELED:
        Toast.makeText(this,"Paymnet Cancel",Toast.LENGTH_LONG).show();
        //finish();
        break;
    case PayPalActivity.RESULT_FAILURE:
        Toast.makeText(this,"Paymnet Failed",Toast.LENGTH_LONG).show();
        String errorID = 
        data.getStringExtra(PayPalActivity.EXTRA_ERROR_ID);
        String errorMessage = 
        data.getStringExtra(PayPalActivity.EXTRA_ERROR_MESSAGE);
        //Toast.makeText(getApplicationContext(),errorMessage, Toast.LENGTH_LONG).show();
        //finish();
        break;
    }
}

So how Activity.RESULT_OK, I would like to send a post to my notify URL so that the codes in the URL can validate payments.

Also, I get that the paykey can return the transaction details. If so, then I will use it to do the verification instead of using notify url. But after doing much reading, I still could not find the answer to return the transaction details, like maybe data.getStringExtra('transaction_id'), data.getStringExtra('merchant_name'), etc.

.

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