If you read the PayPal documentation, the first step is to get a token so you can invoke PayPal APIs. Before you can get the token, you need to create an app in PayPal:

Every app has a Client ID and Secret values that you need to use to get a token.

The Server Code script below sends a request to PayPal to get a token:

var url = "https://api.sandbox.paypal.com/v1/oauth2/token";

var XHRResponse = XHR2.send("POST", url, {
 "parameters": {
 "grant_type": "client_credentials"
 },
 "headers": {
 "Accept": "application/json",
 "Accept-Language": "en_US",
 "Authorization": "Basic clientid:secret_as_base64"
});

Apperyio.response.success(XHRResponse.body, "application/json");

The response looks like this:

{
 "app_id": "APP-80W2.....",
 "access_token": "A101.VyA8xBdNyOY.....",
 "token_type": "Bearer",
 "scope": "https://uri.paypal.com/....",
 "nonce": "2016-04-27T23:15:21Zahq3.....",
 "expires_in": 31284
}

The access_token is what you need to invoke any other PayPal API.

Now that you have the token, you can invoke any PayPal API. In the next script I create a payment (charge):

var url = "https://api.sandbox.paypal.com/v1/payments/payment";

var XHRResponse = XHR2.send("POST", url, {
 "headers": {
 "Content-type": "application/json",
 "Authorization": "Bearer A101.VyA8xBdNyOY....."
 },
 "body": {
 "intent": "sale",
 "payer": {
 "payment_method": "credit_card",
 "funding_instruments": [{
 "credit_card": {
 "number": "4060xxxxxxxxxxxx",
 "type": "visa",
 "expire_month": 06,
 "expire_year": 2019,
 "cvv2": "874",
 "first_name": "Betsy",
 "last_name": "Buyer",
 "billing_address": {
 "line1": "111 First Street",
 "city": "Saratoga",
 "state": "CA",
 "postal_code": "95070",
 "country_code": "US"
 }
 }
 }]
 },
 "transactions": [{
 "amount": {
 "total": "7.47",
 "currency": "USD",
 "details": {
 "subtotal": "7.41",
 "tax": "0.03",
 "shipping": "0.03"
 }
 },
 "description": "This is the payment transaction description."
 }]
 }
});

Apperyio.response.success(XHRResponse.body, "application/json");

The payment example was adopted from PayPal API documentation. 

One important thing you need to do is use a valid credit card number from your test account. Without a valid credit card number, the request will not work. To find your credit card number go to Sandbox > Accounts, expand your account and click Profile. Select the Funding tab and use the credit card and expiration data from there for the test payment.

Once you have a working Server Code script that creates a payment in PayPal, you can use the script in your mobile app. A Server Code script is automatically exposed via a REST API, which you can call from your app. We have many videos on our YouTube channel that show how to use Server Code and other Appery.io backend services.