You don't need to publish an SPF mechanism that authorizes Cvent-planner to send emails on your behalf. That is because the emails sent from their mail servers contains their own domain as the Return-Path header of emails. This means that your SPF record will not be evaluated for SPF, instead emails will be checked against the Cvent-planner's SPF record.
All you need to do is enable DKIM for Cvent-Planner. That will fully authenticate the emails and pass DMARC.
Cvent-planner supports DKIM signing for your custom domain, however, in order to enable it you will have to contact your customer support representative and they will enable it for you. Please see the following link for more information.
Create a free OnDMARC account to test your configuration.