How do I add the payment sandbox like paypal to my project?

What am actually trying to do is to perform a payment with 3 steps:

1) Order Page with PayNow Button for users to click

2) PayPal Payment

3) Redirect to "Payment completed page"

What I want to complete is to have an ID going from step 1 to 3. What I tried to do is to insert this field into the "custom" variable like:


After the PayPal payment, PayPal redirect me to my page:

http://www.mysite.cm/tx=blalba?aaaa

With this URL there is no custom Field. If from API, I contact PayPal to have the sale details, I get nothing related to my custom Field.

OK so here is the problem am having

paypal.Button.render({
//Configure environment
env: 'production', // To test, set to `sandbox`
payment: function () {
// Set up the payment here, when the buyer clicks on the button
},
onAuthorize: function (data, actions) {
// Call your server to execute the payment
if (error === 'INSTRUMENT_DECLINED') {
actions.restart();
}
}
}, '#paypal-button');