Endpoint:
POST /payments/voucher/{payment id}/commit

Authentication:
Type: client
Scopes: [ credit ]

Headers:
Content-Type: application/json
Authentication: Bearer %access_token%

Response body:
{
  "amount": 0
}
CODE

Note that the %access_token% needs to be replaced by the token you received as a response from auth endpoint.

{payment id} have to be included in the endpoint call. This commits the payment from the earlier call and actually makes a change in credit. See Post Payment voucher the how to get the payment id.