Method to schedule a mailing.
https://api.wbsrvc.com/Mailing/Schedule/
POST
| user_key Required |
User Key of the user who initiates the call. Type: string |
| mailing_id Required |
ID of the mailing. Type: integer |
| date Optional |
Date when the mailing is scheduled. If not provided, the mailing will be sent right away. Type: date - yyyy-mm-dd hh:mm:ss |
| client_id Optional |
Client ID of the client in which the mailing is located. Type: integer |