Webhook to Update Lead Field in Marketo

I'm trying to measure how effective our SMS meeting reminders are and if it helps meetings/appointments hold. I can easily set it up so it will only send to 50% of our leads. However, measureing is another story. I know the results will populate who got sent a message and who didn't, but compairing the data in the CRM to see if it held is another story. I've created a field in marketo and our CRM to better run reports. So what my question is, can I create a webhook to update that field for the 50%?