In this doc, "source" indicates the customer/merchant that is controlling the user experience and is receiving the payment method data from the customer. "Target" is the partner that wishes to receive this payment method from the source. If you wish to use this method to essentially copy a payment method within your environment, you will act as both the "source" and the partner".
Note: this example uses XML, but the JSON API is recommended.
$ curl https://core.spreedly.com/v1/receivers.xml \
-u '[SOURCE-ENV-KEY]:[SOURCE-SECRET]' \
-H 'Content-Type: application/xml' \
-d '<receiver>
spreedly