You would either have to use CVENT's API, or use a marketo Webhook and CVENT's HTTP POST format.
The webhook using HTTP POST is probably easier--you'll have to get the format frrom CVENT, then substitue Marketo Tokens for the field information
For ON24, the format is this--I don't know CVENT's format, but they claim to support HTTP POST--you'll have to get the format from them:
http://event.on24.com/utilApp/r?eventid=xxxxxx&sessionid=1&key=xxxxxxxxxxxxxxxxxxxxxxxx&job_title={{lead.Job Title}}&firstname={{lead.First Name}}&lastname={{lead.Last Name}}&email={{lead.Email Address}}
Note that you include the Marketo tokens in the string. job_title is how ON24 idenfies the field, {{lead.Job Title}} is how Marketo's token defines the same field. Omit the "default=edit me" section.
Then build a webhook (on Marketo's ADMIN page), and include this URL. To post the data to CVENT, your Smart campaign would be something like
Smart list: Trigger - Fills out Form
Flow: Call Webhook
Schedule: Activate.
I don't know what CVENT's precise format is, but they claim to support HTTP POST, so you can probably get the inforamtion from their customer support.
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