This POST endpoint is used to initiate a spendback transaction to a provided user token. You can use this endpoint to enable spendback transactions for your users and help them manage their funds more effectively. To use this endpoint, you need to provide the user_token as a parameter in the URL Path, along with the required parameters in the request Body, including client_spendback_id, source_token, source_currency_code and amount. The endpoint will then initiate the spendback transaction, transferring funds from the source token to the user token. The response will contain a JSON object indicating the status of the request and any relevant transaction details.