The detail of the subscription plan you’ve created looks like this:

Notice at the bottom of that screen is a Sample customer URL.
If you click that link, it will bring you here:

As you can see in that screenshot, the sample customer url is:
https://spreedly.com/meresheep-test/subscribers/44763/subscribe/41/screen-name-for-customer-44763
In this case, 44763 is your customer id in your application. 41 is the subscription plan id. screen-name-for-customer-44763 is the screen name for your customer. It’s how the customer will show up in spreedly. This might be just the customer’s email address.
When the user purchases a subscription, they’ll see a page like this:

When they click the Return to Meresheep button, they’ll be redirected to the url you specified for that subscription plan (or the return_url that you specified in the subscribe url).
Now your dashboard has some activity:

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It’s also worth noting that you might want to override the return url for this particular customer’s purchase. In that case, you can pass a return_url parameter in the url like so: